Who is the writer of the book of Al-Musnad?
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The authors of Al-Masaaneed (plural of Al-Musnad) are so many, among whom is Daawood At-Tayaalisi, Abu Ya'laa, Ad-Daarimi, Al-Bazzaar, Imaam Ahmad Ibn Hanbal and others. However, the most well-known amongst them is Imaam Ahmad, and it might be that he is the one you are asking about.
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