Surah 37. As-Saffat, Ayah 112

 The birth of Isaac is announced after the story of sacrifice. So this is a proof that the son who was going to be sacrificed was Ishmael, not Isaac. This is also supported by 11:71, where Sarah is told that she would give birth to Isaac, who would (reach adulthood and) have a son by the name of Jacob.

Isaac was Abraham's second son, born of Sarah, when Abraham was 100 years of age. See n. 4101. He was also blessed and became the ancestor of the Jewish people. See next note.