Muhammad had an elder half-brother, Abdallah, the future al-Ma'mun (), who had been born in September 786. However, Abdallah's mother was a Persian slave concubine, and his pure Abbasid lineage gave Muhammad seniority over his half-brother. Indeed, he was the only Abbasid caliph to claim such descent. Already in 792, Harun had Muhammad receive the oath of allegiance ("bay'ah") with the name of "al-Amīn" ("The Trustworthy"), effectively marking him out as his main heir, while Abdallah was not named second heir, under the name "al-Maʾmūn" ("The Trusted One") until 799. Both brothers were assigned members of the powerful Barmakid family as tutors: al-Amin's tutor was al-Fadl ibn Yahya, while al-Ma'mun's was Ja'far ibn Yahya.