Al-Farḥayānī and other leading ḥadīth masters said, “The most knowledgeable person of his generation concerning the hidden defects (ʿilal) of ḥadīth was ʿAlī.”

Yaʿqūb al-Fasawī states in his History (1): “Bakr ibn Khalaf related to me:

‘I arrived in Mecca where there was a young memorizer who used to review with me the Musnad (2) and all of its various chains. I asked him, “Where did you get this from?”
He replied, “I will tell you. In the days of Sufyān I asked ʿAlī to narrate the Musnad to me. He said, ‘You already know that your aim is only to revise it. If you guarantee that you will use it for review and will not mention my name, I will do it.’
So I gave him that guarantee and kept attending his sessions, and he proceeded to relate to me—by heart—the very material I am now rehearsing with you.”’

Al-Fasawī continues: “I mentioned this incident to someone who habitually kept company with ʿAlī, and he said:

‘I heard ʿAlī say, “I was absent from Basra on a journey to Yemen—I think he said for three years—while my mother was still alive. When I returned, she said, ‘My son, so-and-so is your friend and so-and-so is your enemy.’
I asked, ‘How do you know that, Mother?’
She answered, ‘So-and-so and so-and-so—including Yaḥyā ibn Saʿīd—used to come, greet me, console me, and say: Be patient; when he returns, Allah will gladden you with what you will see. From this I realized they were friends.
As for so-and-so and so-and-so, whenever they came they would say: Write to him and press him so that he will come back.’”’

Al-ʿAbbās ibn ʿAbd al-ʿAẓīm—or someone else—told me that ʿAlī said:

‘I had compiled the Musnad exhaustively, arranging it according to its various transmission routes. I wrote it on parchments, placed it in a large chest, left it at home, and then set off on this journey. When I returned…’