'Abdullah ibn al-Hasan ibn al-Hasan ibn 'Ali ibn abi Talib (70/690-145/762) was a descendent of the Prophet (s.a.w) via Imam Hasan (a.s) (i.e. a Hasani sayyid). Imam Ali(a.s) and Lady Fatimah(a.s) had two sons survive to adulthood: Hasan (a.s), the eldest son, and Husayn(a.s), the younger of the two. The Imamate did not continue via the descendents of (the second Imam) Hasan (a.s) but from (the third Imam) Husayn (a.s). The fourth Imam, and son of Imam Husayn(a.s), Ali ibn al-Husayn (a.s), married his first cousin, the daughter of Imam al-Hassan (a.s). Their son, Mohammad ibn 'Ali (a.s), was the fifth Imam, “uniting” the Hassani and Husayni lines of Imamate.
'Abdullah ibn al-Hasan tried to put forth his son, Muhammad whom he referred to as al-Mahdi (the rightly guided one) and Dhu al-nafs al-zakiyyah (possessor of the pure soul) appointed Caliph during the times of the 'Abbasids on the basis of kinship to the Ahl al-Bayt. Both 'Abdullah ibn al-Hasan and the 'Abbasids claimed a right to leadership based on lineage, the former through Imam Hasan (a.s), the latter through 'Abbas (a.s), the Prophet's uncle (s.a.w). Imam al-Sadiq(a.s) knew that nothing would come of Mohammad ibn 'Abdullah's uprising because nothing was written of his rule in Mushaf Fatimah, which mentioned the names of all future rulers.