He was one of the leading Qur'anic commentators and a translator of the generation after that of the Prophet Muhammad and his Companions. He is the first to compile a written exegesis of the Qur'an, in which he stated It is not permissible for one who holds faith in Allah and the Day of Judgment to speak on the Qur'an without learning classical Arabic. He is said to have studied under Amir al-Mu'minin 'Ali ibn Abi Talib until his martyrdom. At that point, he began to study under Ibn Abbas, a companion of the Prophet known as the father of Qur'anic exegesis. Mujahid ibn Jabr was known to be willing to go to great lengths to discover the true meaning of a verse in the Qur'an, and was considered to be a well-travelled man. However, there is no evidence he ever journeyed outside of the Arabian Peninsula.