Muslims believe that it was Ishmael, in contrast to the Taurat's record that it was Isaac. However, this was not a unanimous opinion. Those early Muslim commentators who believe that it was Ishmael include al-Zamkhshari, al-Baidhawi, al-Tabari and Ibn al-Athir. This tradition was often used: "Umar b. Abd-Al-Aizi asked a Jew converted to Islam about the difference of opinion and he answered: "The 'dhabhi' is Ismail; the Jews know this also, but they are jealous of you, they say it was Ishak." (Shorter Encyclopaedia of Islam, p.175) . On the other hand, Al-Tha`labi expressly emphasises the Ashab (companions) and Tabi`un (successors of the companions), from `Umar Al-Khattab to Ka`b Al-Ahbar, believe that the person sacrificed was Isaac. (ibid, p.175)

A scholarly investigation into the issue is

Reuven Firestone
Journeys in Holy Lands:
The Evolution of the Abraham-Ismael Legends in Islamic Exegesis
State University of New York Press, 1990, 265p, $24.95, ISBN 0-7914-0332-7

and in the appendix all the Islamic witnesses are listed, those who say Isaac and those who say Ishmael. It is pretty even in the outcome.