In The Name of Allah, The Most Beneficent, The Most Merciful
Once Hasan bin Fazal asked Imam Ali Reza (a.s.) why the Holy Prophet (s.a.w.a.) was called Abul Qasim. He said: “Because the name of one of his sons was Qasim.” Hasan asked: “Do your honor, not regard me worthy of being informed more than this?”
The Imam (a.s.) said: “Why not? Perhaps you are not aware that the Holy Prophet (s.a.w.a.) has said: I and Ali are two fathers of this Ummah.” I said: “I do know that.” Then he asked again: “Do you know that the Holy Prophet (s.a.w.a.) is the father of this entire Ummah?” I said: “Yes, I know.” He asked further: “Do you know that Ali (a.s.) is the distributor of Paradise and Hell?” I said: “Yes.” He said: “The Holy Prophet (s.a.w.a.) is the father of the distributor of Paradise and Hell and hence the Almighty Allah gave him the title of Abul Qasim (father of the distributor).” I asked: “In what way is his being the father?” He replied: “It means that the admiration and love of the Holy Prophet (s.a.w.a.) for the entire Ummah is like the affection of a father for his children. And Ali (a.s.) is the best individual in Prophet’s
Ummah. Similarly after the Holy Prophet (s.a.w.a.), the love of Ali for the Ummah is like Prophet’s love because he is his legatee and the leader of the Ummah.
Hence it is said by him that: I and Ali are two fathers of this Ummah.” Then the Imam said: “One day the Holy Prophet (s.a.w.a.) went to the pulpit and said: If a man dies leaving behind some debt and family members, the responsibility of his debt and maintenance of his family members is on me and if a man dies and leaves behind him his heirs and also wealth, then that wealth belongs to his heirs.”
Because of this the Holy Prophet (s.a.w.a.) has the right over his Ummah which is higher than their own right over their lives. Likewise, Ali’s right over the lives of the Ummah, after the Holy Prophet (s.a.w.a), was higher than the Ummah’s right over their own lives.”
Ref: Hayat Al Quloob