Usul al-Kafi
Book 4: The Book of Divine Proof

Chapter 63
The continuity of Imamate
419. 1. Imam Jafar as-Sadiq (p): Imamate will never be conferred on two brothers after Imam al-Hasan (p) and Imam al-Husayn (p). The Imamate continued from Imam Zainul Abideen (p). God, the Glorious, the Exalted, says: ...But kindred by blood have prior rights against each other in the Book of God... (Q, 8:75).
420. 5. Imam Jafar as-Sadiq (p) was asked: Who will be the next Imam after your demise?
The Imam (p) pointed towards Imam Musa al-Kazim (p).
The Imam (p) was again asked: And if the survivors include an elder brother and a son, who will be the Imam?
The Imam (p): The son and then his son and so on.