Muslims have killed billions of animals in the past 1400 years, but they don't even know if it was Ishaaq who was slaughtered or Ismael.

There are 131 traditions, which claim Ishaaq was the son who was sacrificed. But later Hadith Fabrication Factory by Islam apologists fabricated 133 traditions in order to counter the first Statement and they claimed it was Ismael who was sacrificed. Through this, they wanted to give preference to Muhammad and his ancestor (Ismael) over Jews and their ancestor Ishaaq (link). 

Imam Qurtabi (under the commentary of Verse 37:102) recorded the name of Sahaba and Tabaeen who declared Ishaaq to be Zabih-Ullah i.e. the son who was sacrificed (link):

وٱختلف العلماء في المأمور بذبحه. فقال أكثرهم: الذبيح إسحاق. وممن قال بذلك العباس بن عبد المطلب وٱبنه عبد اللّه وهو الصحيح عنه. روى الثوريّ وٱبن جريج يرفعانه إلى ٱبن عباس قال: الذبيح إسحاق. وهو الصحيح عن عبد اللّه بن مسعود أن رجلاً قال له: يا بن الأشياخ الكرام. فقال عبد اللّه: ذلك يوسف بن يعقوب بن إسحاق ذبيح اللّه بن إبراهيم خليل اللّه صلى اللّه عليهم وسلم ـ. وقد روى حماد بن زيد يرفعه إلى رسول اللّه صلى الله عليه وسلم قال: " إن الكريم ابن الكريم ابن الكريم ابن الكريم يوسف بن يعقوب بن إسحاق بن إبراهيم صلى الله عليه وسلم " وروى أبو الزبير عن جابر قال: الذبيح إسحاق. وذلك مروي أيضاً عن عليّ بن أبي طالب رضي اللّه عنه. وعن عبد اللّه بن عمر: أن الذبيح إسحاق. وهو قول عمر رضي اللّه عنه. فهؤلاء سبعة من الصحابة. وقال به من التابعين وغيرهم عَلْقَمة والشّعبي ومجاهد وسعيد بن جُبير وكعب الأحبار وقتادة ومسروق وعِكرمة والقاسم بن أبي بَزَّة وعطاء ومقاتل وعبد الرحمن بن سابط والزهريّ والسديّ وعبد اللّه بن أبي الهذيل ومالك بن أنس، كلهم قالوا: الذبيح إسحاق. وعليه أهل الكتابين اليهود والنصارى، واختاره غير واحد منهم النحاس والطبري وغيرهما.

There is a difference of opinion among the scholars who was sacrificed. The MAJORITY of them said that it was Ishaaq .... (Then Imam Qurtabi recorded the name of Sahaba and Tabaeen who declared Ishaaq to be Zabih-Ullah.
They were 7 companions:
(1) Ibn 'Abbas (2) 'Ali ibn Abi Talib (3) 'Abdullah Ibn Masood (4) Jabir bin 'Abdullah Ansari (5) Umar Ibn Khattab (6) Abdullah Ibn Umar (7) Abbas bin Abdul Muttalib.
While the following 16 Successors (i.e. the Muslim generation after Sahaba) also considered Ishaaq to be Zabih-Ullah:
(1) Alqama (2) Sh'ubi (3) Mujahid (4) Saeed bin Jubair (5) K'ab al-Ahbar (6) Qatadah (7) Masrooq (8) 'Ikrimah (9) Qasim bin Abi Baza (10) 'Atta (11) Maqatil (12) Abdul Rehman bin Sabit (13) Zuhri (14) Suddi (15) 'Abdullah bin Hazil (16) Imam Malik ibn Anas. While scholars like Nahas and Tabari also declared Ishaaq to be Zabih-Ullah). 

After that, Imam Qurtabi also recorded the name of Sahaba and Tabaeen who declared Ismael to be Zabih-Ullah.

وقال آخرون: هو إسماعيل. وممن قال ذلك أبو هريرة وأبو الطفيل عامر بن واثلة. وروي ذلك عن ٱبن عمر وٱبن عباس أيضاً، ومن التابعين سعيد بن المسيّب والشّعبي ويوسف بن مِهْران ومجاهد والربيع بن أنس ومحمد بن كعب القُرَظيّ والكلبي وعلقمة.
(Imam Qurtabi then recorded the name of Sahaba and Tabaeen who declared Ismael to be Zabih-Ullah.
Those were 4 Sahaba:
(1) Abu Hurairah (2) Abu Tufail 'Aamir bin Wasila (3) Ibn Umar (4) Ibn Abbas.
And there were 8Tabaeen, who are as under:
(1) Sa'eed bin al-Musayyab (2) Sh'ubi (3) Yousuf bin Mahran (4) Mujahid (5) Rabi bin Anas (6) Muhammad bin Kaab al-Qurzi (7) Kalbi (8) Alqama)

In the end, Imam Qurtabi wrote his own decision in the following words (link):

وهذا القول أقوى في النقل عن النبي صلى الله عليه وسلم وعن الصحابة والتابعين.
The saying (i.e. Ishaaq was Zabih-Ullah) has been narrated through stronger traditions from the Prophet and Sahaba and Successors (i.e. Tabaeen).

Conclusion:

A funny situation arises while there are traditions from Ibn Abbas, 'Abdullah Ibn Umar, Sh'ubi, Mujahid and Alqama who declare Ishaaq to be Zabih-Ullah, but at the same time, there are traditions where these same people are declaring Ismael to be Zabih-Ullah (i.e. the one who was slaughtered). 

How is it possible?

These are the contradictions which completely expose Ahadith, Ilm-ul-Hadith, and Muslim factories of fabrication of Ahadith.