Ibn Abī Ḥātim said, “ʿAbd Allāh wrote to me the legal questions (masāʾil) of his father, along with the ʿilal (hidden defects) of hadith. (1)
Abū al-Ḥusayn Aḥmad ibn Jaʿfar ibn al-Munādī said, “There was no one on earth who transmitted more from his father than did ʿAbd Allāh ibn Aḥmad. He heard from him the Musnad, which numbers thirty thousand reports, and the Tafsīr, which comes to one hundred and twenty thousand; of this he personally heard eighty thousand, and the rest he acquired through wijādah. (2) He also heard al-Nāsikh wa-l-mansūkh, al-Tārīkh, Ḥadīth Shuʿbah, al-Muqaddam wa-l-muʾakhkhar fī Kitāb Allāh, Jawābāt al-Qurʾān, the large work on Manāsik and the smaller one, along with other compilations and the hadiths of the various shaykhs.
He added, “We have always seen our senior teachers testify to his expertise in rijāl, the ʿilal of hadith, personal names and kunyas, and his unswerving dedication to the pursuit of hadith in Iraq and elsewhere. They reported that their predecessors had likewise acknowledged this, so much so that some of them went to great lengths in praising his knowledge and in asserting that the amount he heard of hadith exceeded even that of his father. (3)"
(1) Al-Jarh wa al-Taʿdil, vol. 5, p. 7.
(2) Wajada (written transmission): when a student of hadith discovers reports in the handwriting of their transmitter—whether he has met or heard from him or not—or finds reports in books by well-known authors. He may not narrate them directly from those transmitters; rather he must say, “I found it in the handwriting of so-and-so,” once he has identified and trusted the script, or say, “So-and-so said,” and the like.
The Musnad of Ahmad contains many examples of this; his son ʿAbd Allah relates them, stating, “I found this in my father’s own handwriting in his book.”
More than one critical scholar has maintained that acting upon wajada is obligatory when one is confident about what he has found—either that the report is in the handwriting of a known shaykh, or that the book consulted is certainly the work of its attributed author. In addition, the author must be reliable and the isnad of the report sound before the narration becomes binding in practice.
When these conditions are met, a well-verified wajada is no less trustworthy than any type of ijaza (authorised transmission). Indeed, the principal source-books of hadith and other Islamic sciences have reached us through successive wajada transmissions and through various ancient, dependable manuscripts.
(3) See Tahdhib al-Tahdhib, vol. 5, pp. 142–143.