Short answer
Most Muslim scholars hold that the son Ibrahim was commanded to sacrifice was Ismail (Ishmael), his firstborn. The Quran tells the story of the sacrifice and only afterwards says "We gave him good tidings of Isaac", which shows Ishaq was a separate, later gift. Some early scholars said it was Ishaq, but the stronger view is Ismail.
The relied-upon view among Sunni scholars, including the Hanafi commentators, is that the son of the sacrifice (al-dhabih) was Ismail. It is not a point of creed that makes anyone a disbeliever, since some early authorities held the other view, and both sons are honoured prophets.
The four schools agree on this.
Explanation
Surah al-Saffat tells the story. Ibrahim asked for a righteous child, and "We gave him good tidings of a forbearing boy" (37:101). When the boy was old enough to work with him, Ibrahim told him of his dream that he was sacrificing him. The son replied: "O my father, do as you are commanded. You will find me, if Allah wills, of the steadfast" (37:102). When both submitted, Allah called out to Ibrahim and "ransomed him with a great sacrifice" (37:107).
Only after the story ends does the Quran say: "And We gave him good tidings of Isaac, a prophet from among the righteous" (37:112). Most commentators reasoned that the son of the sacrifice must therefore be the son mentioned before, Ismail. Another verse says Sarah was given "good tidings of Isaac and after Isaac, Jacob" (11:71); since Ishaq was promised a son of his own, a command to sacrifice him as a boy would not fit. The rites of Hajj in Makkah, where Ismail lived, also carry the memory of the sacrifice.
Some Companions and early scholars said the son was Ishaq, and some later historians followed them. Commentators such as Ibn Kathir weighed the evidence and judged Ismail the stronger view, saying the opposite view came largely from Israelite reports. The Bible names Isaac, which is one reason the question is asked.
Every year Muslims remember this obedience through qurbani. Read the stories of Ibrahim, Ismail and Ishaq.
The evidence
Quran, Surah As-Saaffaat 37:101
فَبَشَّرْنَٰهُ بِغُلَٰمٍ حَلِيمٍۢ ﴿١٠١﴾
So We gave him good tidings of a forbearing boy.
The son given before the sacrifice.
Quran, Surah As-Saaffaat 37:107
وَفَدَيْنَٰهُ بِذِبْحٍ عَظِيمٍۢ ﴿١٠٧﴾
And We ransomed him with a great sacrifice,
The son was ransomed.
Quran, Surah As-Saaffaat 37:112
وَبَشَّرْنَٰهُ بِإِسْحَٰقَ نَبِيًّۭا مِّنَ ٱلصَّٰلِحِينَ ﴿١١٢﴾
And We gave him good tidings of Isaac, a prophet from among the righteous.
Ishaq is announced after the story, as a separate son.
Where the ruling is recorded
- Tafsir Ibn Kathir, Commentary on Surah al-Saffat, verses 99 to 113. Weighs the two views and prefers that the son of the sacrifice was Ismail.
- Ma'arif al-Quran (Mufti Muhammad Shafi), Commentary on Surah al-Saffat. Explains the story and the view that the son was Ismail.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.