Sahih Bukhari, Hadith 2731:

... ('Urwa, the ambassador of Pagan Quraysh, said to Muhammad) O Muhammad! Won’t you feel any scruple in extirpating your relations? Have you ever heard of anyone amongst the Arabs extirpating his relatives before you? On the other hand, if the reverse should happen, (nobody will aid you, for) by Allah, I do not see (with you) dignified people, but people from various tribes who would run away leaving you alone.”

Upon that, Abu Bakr replied with these dirtiest possible abusive words:

امْصُصْ بَظْرَ اللاَّتِ

Go suck the clitoris of (your female goddess) Al-lat.

Meaning of word امْصُصْ : Suck (Dictionaries 123)

Meaning of word بَظْرَ : Clitoris - Small erectile part the female genitals at the anterior end of the vulva (Dictionaries 1234)

Meaning of word  اللاَّتِ : al-Lat - Name of female goddess of Pagans

Note: Prophet Muhammad neither stopped Abu Bakr from uttering those abusive words, nor punished him for any blasphemy. But contrary to this, he supported him by keeping quiet. 

The largest Islamic Website on Internet is IslamWeb.Net. They gave the following excuse for those abusive words from Abu Bakr, and reaction of prophet Muhammad upon that (link):

... (do you) makes no difference between the One True God (i.e. Allah) and the false gods (of non-Muslims);  ...  insulting the god of non-Muslims is an act by which one gets closer to Allaah and it is an act of obedience۔۔۔ (Then this website Islamweb.net further recorded the opinions of early Muslim Scholars like Ibn Taymiyyah, Ibn Hajar and Ibn Qayyim who all praised this usage of abusive wordy from Abu Bakr, and declared it to be part of Sharia, and told Muslims to abuse the gods of non-Muslims in this way).

There was no insult present in the words of 'Urwah (i.e. the ambassador of polytheists of Quraysh), and he was only telling his opinion as Muslims had already run away before in the battle of Uhud, leaving Muhammad alone. 

It is human nature when people lose their argument (especially those who are not well-civilized), then they become aggressive and abusive. Same thing happened with Abu Bakr and Muhammad and they became aggressive and abusive towards 'Urwah. And especially considering that 'Urwah was an ambassador, then only a diplomatic civilized argument could be given in response to his argument instead of extreme insult and abuse.

Distortion of those abusive words of Abu Bakr by dishonest Muslim Translators

These words of Abu Bakr are so dirty, that all Muslim Translators of Sahih Bukhari distorted the translation, and hid the actual words of Abu Bakr.  

The most common English Translator of Sahih Bukhari is Mohsin Khan. Let us look at his distortion. He translated these words as (link):

امْصُصْ بَظْرَ اللاَّتِ

Hearing that, Abu Bakr abused him.

The distortion here is obvious as it completely removes what the actual abuse was and instead leaves the reader with no clue as to what the comment was nor the severity of it.

Aisha Bewley (Another Translator) goes another route, instead changing the bodily anatomy that Abu Bakr orders Urwah to suck:

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Even worse, the Spanish version replaces Abu Bakr’s words with entirely different words. The passage is translated as “Abû Bakr le dijo: ‘Húndete en la ignominia!’” translates into English as “Abu Bakr said to him: ‘Bury yourself in ignominy!’”

But however, we have one honest translator in the form of Michael Fishbein:

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Islam Apologists: But Quran forbade to abuse the gods of non-Muslims:

Islam apologists come up with the excuse how could Abu Bakr abuse 'Urwa in presence of Muhammad, while Quran forbade to abuse the gods of non-Muslims. 

Quran 6:108:

Do not revile those who invoke others apart from God, lest they begin to revile God out of malice and ignorance.

Answer: 

This verse 6:108 of Suran al-Anam was revealed in the last years of Meccan life, when Muslims were weak and they feared the power of pagans of Mecca. While the incident of 'Urwa and Abu Bakr happened many years later in the Medinan life (during the incident of Hudaybiah in 6th Hijri year), when Muslims had already become stronger and they had no fear of Meccans.

Whole Quran is witness that initially in the Meccan life, when Muslims were weak, then no Quranic verses were revealed about Qataal (killing) and Jihad (holy war).

But later in the Medinan life, when Muslims became stronger, then Quran talked only about killing and Jihad and threatening the Kuffars.

Please read out detailed article upon this: When Islam was weak then Quranic Verses taught peace, When Islam became stronger, then Quranic Verses taught violence, cruelty, coercion, and intolerance.

Are Muslims now going to apologize in name of Quran and Muhammad and Abu Bakr for doing all these blasphemous acts? 

Quranic Verses which are insulting the non-Muslims

(Quran 68:10-13) Heed not the type of despicable men,- ready with oaths, A slanderer, going about with calumnies, (Habitually) hindering (all) good, transgressing beyond bounds, deep in sin, Violent (and cruel),- with all that, base-born (bastard),-

The Quran used all these bad words for Walid bin al-Mughirah. He didn’t accept that Muhammad was any prophet, but he said that Muhammad had lost his senses. Upon that Quran called him all these names including “bastard”.

And then writer of Quran (i.e. Muhammad) abused Abu Lahab in these words:

(Quran 111:1) May the hands of Abu Lahab perish! May he too perish! ..

And then Muhammad changed the name of “Amr Ibn Hisham” in order to abuse him and started calling him “Abu Jahl (i.e. Father of Ignorance)”. This Amr Ibn Hisham was a very wise person, and Quraysh used to call him “Abu Hakam (i.e. Father of Wisdom)”. But he opposed Muhammad, thus Muhammad started abusing him by calling him “Abu Jahl”.

(Quran 2:89) فَلَعْنَةُ اللَّـهِ عَلَى الْكَافِرِينَ
Allah's curse is on the unbelievers.
(Quran 2:161) نَّ الَّذِينَ كَفَرُوا وَمَاتُوا وَهُمْ كُفَّارٌ أُولَـٰئِكَ عَلَيْهِمْ لَعْنَةُ اللَّـهِ وَالْمَلَائِكَةِ وَالنَّاسِ أَجْمَعِينَ
Those who reject Faith, and die rejecting,- on them is Allah's curse, and the curse of angels, and of all mankind;
(Quran 7:179) لَقَدْ ذَرَأْنَا لِجَهَنَّمَ كَثِيرًا مِّنَ الْجِنِّ وَالْإِنسِ لَهُمْ قُلُوبٌ لَّا يَفْقَهُونَ بِهَا وَلَهُمْ أَعْيُنٌ لَّا يُبْصِرُونَ بِهَا وَلَهُمْ آذَانٌ لَّا يَسْمَعُونَ بِهَا أُولَـٰئِكَ كَالْأَنْعَامِ بَلْ هُمْ أَضَلُّ أُولَـٰئِكَ هُمُ الْغَافِلُونَ
Already have We urged unto hell many of the jinn and humankind, having hearts wherewith they understand not, and having eyes wherewith they see not, and having ears wherewith they hear not. These are as the cattle - nay, but they are worse than cattle
(Quran 62:5) مَثَلُ الَّذِينَ حُمِّلُوا التَّوْرَاةَ ثُمَّ لَمْ يَحْمِلُوهَا كَمَثَلِ الْحِمَارِ يَحْمِلُ أَسْفَارًا بِئْسَ مَثَلُ الْقَوْمِ الَّذِينَ كَذَّبُوا بِآيَاتِ اللَّـهِ وَاللَّـهُ لَا يَهْدِي الْقَوْمَ الظَّالِمِينَ
The likeness of those who were charged with the Taurat, then they did not observe it, is as the likeness of the ass (donkey) bearing books, evil is the likeness of the people who reject the communications of Allah; and Allah does not guide the unjust people.
(Quran 2:171) وَمَثَلُ الَّذِينَ كَفَرُوا كَمَثَلِ الَّذِي يَنْعِقُ بِمَا لَا يَسْمَعُ إِلَّا دُعَاءً وَنِدَاءً صُمٌّ بُكْمٌ عُمْيٌ فَهُمْ لَا يَعْقِلُونَ
And the parable of those who disbelieve is as the parable of one who calls out to that which hears no more than a call and a cry; deaf, dumb (and) blind, so they do not understand.
(Quran 5:60) قُلْ هَلْ أُنَبِّئُكُم بِشَرٍّ مِّن ذَٰلِكَ مَثُوبَةً عِندَ اللَّـهِ مَن لَّعَنَهُ اللَّـهُ وَغَضِبَ عَلَيْهِ وَجَعَلَ مِنْهُمُ الْقِرَدَةَ وَالْخَنَازِيرَ وَعَبَدَ الطَّاغُوتَ أُولَـٰئِكَ شَرٌّ مَّكَانًا وَأَضَلُّ عَن سَوَاءِ السَّبِيلِ
Say, "Should I tell you who will receive the worst punishment from God? Those whom God has condemned, afflicted with His anger, made apes out of them, swine and worshippers of Satan, will have the worst dwelling and will wander far away from the right path."
(Quran 7:176) وَلَوْ شِئْنَا لَرَفَعْنَاهُ بِهَا وَلَـٰكِنَّهُ أَخْلَدَ إِلَى الْأَرْضِ وَاتَّبَعَ هَوَاه فَمَثَلُهُ كَمَثَلِ الْكَلْبِ إِن تَحْمِلْ عَلَيْهِ يَلْهَثْ أَوْ تَتْرُكْهُ يَلْهَث ذَّٰلِكَ مَثَلُ الْقَوْمِ الَّذِينَ كَذَّبُوا بِآيَاتِنَا فَاقْصُصِ الْقَصَصَ لَعَلَّهُمْ يَتَفَكَّرُونَ 
And if We had pleased, We would certainly have exalted him thereby; but he clung to the earth and followed his low desire, so his parable is as the parable of the dog; if you attack him he lolls out his tongue; and if you leave him alone he lolls out his tongue; this is the parable of the people who reject Our communications;
(Quran 47:12) إِنَّ اللَّـهَ يُدْخِلُ الَّذِينَ آمَنُوا وَعَمِلُوا الصَّالِحَاتِ جَنَّاتٍ تَجْرِي مِن تَحْتِهَا الْأَنْهَارُ وَالَّذِينَ كَفَرُوا يَتَمَتَّعُونَ وَيَأْكُلُونَ كَمَا تَأْكُلُ الْأَنْعَامُ وَالنَّارُ مَثْوًى لَّهُمْ
Surely Allah will make those who believe and do good enter gardens beneath which rivers flow; and those who disbelieve enjoy themselves and eat as the beasts eat, and the fire is their abode.
(Quran 74:51) فَمَا لَهُمْ عَنِ التَّذْكِرَةِ مُعْرِضِينَ كَأَنَّهُمْ حُمُرٌ مُّسْتَنفِرَةٌ  
What is then the matter with them (Kuffar), that they turn away from the admonition. As they were frightened donkeys.
(Quran 7:166) فَلَمَّا عَتَوْا عَن مَّا نُهُوا عَنْهُ قُلْنَا لَهُمْ كُونُوا قِرَدَةً خَاسِئِينَ
Therefore when they revoltingly persisted in what they had been forbidden, We said to them: Be apes, despised and hated.
(Quran 98:6) إِنَّ الَّذِينَ كَفَرُوا مِنْ أَهْلِ الْكِتَابِ وَالْمُشْرِكِينَ فِي نَارِ جَهَنَّمَ خَالِدِينَ فِيهَا أُولَـٰئِكَ هُمْ شَرُّ الْبَرِيَّةِ
The disbelievers among the People of the Book and the pagans will dwell forever in hell; they are the worst of all creatures.
(Quran 2:99) وَلَقَدْ أَنزَلْنَا إِلَيْكَ آيَاتٍ بَيِّنَاتٍ وَمَا يَكْفُرُ بِهَا إِلَّا الْفَاسِقُونَ 
We have sent down to you clear revelations: no one can deny them except the evil transgressors.
(Quran 9:28) يَا أَيُّهَا الَّذِينَ آمَنُواْ إِنَّمَا الْمُشْرِكُونَ نَجَسٌ فَلاَ يَقْرَبُواْ الْمَسْجِدَ الْحَرَامَ بَعْدَ عَامِهِمْ
O Believers, the pagans are filthy and unclean. Do not let them come near to the Sacred Mosque after this year.
(Quran 8:55) إِنَّ شَرَّ الدَّوَابِّ عِندَ اللَّـهِ الَّذِينَ كَفَرُوا فَهُمْ لَا يُؤْمِنُونَ
Surely the vilest of animals in Allah's sight are those who disbelieve

(Quran 2:142) سَيَقُولُ السُّفَهَاءُ مِنَ النَّاسِ مَا وَلاَّهُمْ عَن قِبْلَتِهِمُ الَّتِي كَانُواْ عَلَيْهَا قُل لِّلّهِ الْمَشْرِقُ وَالْمَغْرِبُ يَهْدِي مَن يَشَاءُ إِلَى صِرَاطٍ مُّسْتَقِيمٍ
The fools (pagans, hypocrites, and Jews) among the people will say, "What has turned them (Muslims) from their Qiblah [prayer direction (towards Jerusalem)] to which they were used to face in prayer." Say, (O Muhammad SAW) "To Allah belong both, east and the west. He guides whom He wills to a Straight Way." 

Conclusion:

There should be absolutely no punishment for blasphemy. 

Yes, we have to discuss issues with each other while avoiding any harsh words. Nevertheless, humans are not perfect. It is a part of human "nature" that one gets emotional/angry during debate and utter harsh words. 

Even Allah and Muhammad and Abu Bakr were also not free of these human emotions/anger, and thus they also uttered such harsh and abusive words for the opponents. 

(Credit: The Islam Issue)