I entered the room of Aḥmad ibn Ḥanbal and he asked, “Who is with you?”
I replied, “Muḥammad ibn al-Nuʿmān ibn ʿAbd al-Salām.”
He did not recognise the name, so I mentioned several others, yet he did not know them either.
Then he said, “Is Abū Masʿūd among you?”
I answered, “Yes.”
He remarked, “Today—so far as I recall—there is no man whose hair is still black who knows the musnad reports (hadiths traced back to the Prophet) of the Messenger of Allah ﷺ better than he.” ⦗1⦘

Abū ʿArūbah al-Ḥarrānī said, “Abū Masʿūd al-Iṣfahānī is on a par with Abū Bakr ibn Abī Shaybah in memorisation, and with Aḥmad ibn Sulaymān al-Ruhāwī in precision.” ⦗2⦘

It is related that Aḥmad ibn al-Furāt first came to Iṣfahān with no books at all; he nevertheless dictated several thousand hadith from memory. When his books later arrived, they were compared with what he had dictated and differed only in a handful of places.

Aḥmad ibn Maḥmūd ibn Ṣabīḥ reported: “I heard Abū Masʿūd al-Rāzī say, ‘I wish I could be slain out of love for Abū Bakr and ʿUmar.’”

Abū Bakr al-Khaṭīb said, “Abū Masʿūd was one of the ḥuffāẓ (master memorisers). He travelled extensively, visiting in one journey Basra, Kūfa, the Ḥijāz, Yemen, Syria, Egypt, and al-Jazīrah. He later came to Baghdad, reviewed hadith there with its leading scholars in the presence of Aḥmad ibn Ḥanbal, and Aḥmad used to give him precedence.” ⦗3⦘

Abū Aḥmad ibn ʿAdī said, “I know of no objectionable report in Abū Masʿūd al-Rāzī’s narrations; he was a man noted for truthfulness and strong retention.” ⦗4⦘