He heard from: ʿAlī ibn al-Jaʿd, Aḥmad ibn Ḥanbal, Yaḥyā al-Ḥimmānī, Khalaf ibn Hishām, Yaḥyā ibn Maʿīn, Ibn Abī Shaybah, his own father, and their contemporaries. He authored several works, and his name became well known.

A great many narrated from him, among them: Abū Sahl ibn Ziyād, Jaʿfar al-Khuldī, Daʿlaj al-Sijzī, Abū Bakr al-Shāfiʿī, Abū al-Qāsim al-Ṭabarānī, Abū Bakr ibn Isḥāq al-Ṣibghī, and the judge Abū Ṭāhir al-Dhuhlī, the qāḍī of Egypt.

Al-Ṣibghī said, “Among the guardians of hadith we have not seen anyone more imposing or more scrupulous than Mūsā ibn Hārūn.” (1)

The ḥāfiẓ ʿAbd al-Ghanī ibn Saʿīd remarked, “The finest critic of the hadith of the Messenger of Allah ﷺ in his era was ʿAlī ibn al-Madīnī; in his time it was Mūsā ibn Hārūn; and in his age it was al-Dāraquṭnī.”

Abū ʿAbd Allāh al-Ḥākim said: I heard Abū Sahl ibn Ziyād say, “Ismāʿīl the Judge would seat Mūsā ibn Hārūn beside him on his couch and have him review everything that was read to him—that is, so that he could verify it for him—despite Ismāʿīl’s own reliability and eminent standing in knowledge and hadith. However, he had grown old and was approaching ninety, and he feared that, after his footing had been firm, it might slip.”

Abū Bakr al-Khaṭīb said, “Mūsā was reliable, a ḥāfiẓ.” (2)

It was also said that Mūsā performed the Hajj frequently: he would reside in Baghdad for a year, then make the pilgrimage and stay near the Sacred Precincts for a year. I think he traded during those journeys.