Zayd ibn Ali, a great-grandson of Ali through Husayn, led an uprising against Umayyad rule in Kufa in 740 CE and was killed in the fighting, and the branch of Shia Islam that continued to follow his line rather than the line the Twelvers recognize took its name, Zaidiyya, directly from him. The Musnad attributed to him gathers hadith and legal rulings said to have been transmitted through Zayd from his father, Ali ibn al-Husayn (Zayn al-Abidin), and grandfather, Husayn ibn Ali, tracing the chain back to Ali himself, and organizes the material by legal topic in a style closer to a Sunni fiqh manual than to the theological weight Twelver collections like Kafi place on doctrine.
The alternative title by which some manuscripts circulate, Majmu al-Fiqh, reflects that practical, law-focused character directly. As the earliest surviving text closely associated with a named Zaidi Imam, it occupies a place in Zaidi legal tradition roughly comparable to the role the Four Books play for Twelver Shia Islam, a founding reference later scholarship built directly on top of. Zaidi legal reasoning built on the Musnad also draws, more than Twelver or Sunni jurisprudence typically does, on independent scholarly judgment, ijtihad, a methodological emphasis some scholars trace directly back to the practical, case by case character of the collection itself.