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

Chapter 66
Imamate of Imam al-Hasan (p): the second Imam

426. 5. Imam Muhammad al-Baqir (p) said that Imam Ali (p) nominated Imam al-Hasan (p) as his successor, and made Imam al-Husayn (p), Muhammad Hanafiah, and his other sons and members of the family, and prominent Shias as witnesses to this nomination. Then Imam Ali (p) handed over the Book and the relics (sacred personal effects) to Imam al-Hasan (p) and said: The Prophet Muhammad (p) has ordered me to appoint you as my successor and to hand over the Books and relics to you just as the Prophet Muhammad (p) had handed over these things to me. I have also been ordered to direct you to hand them over to your brother Imam al-Husayn (p) just before death claims you.
Then, Imam Ali (p) turned towards his son Imam al-Husayn (p) and holding the hand of Imam Zainul Abideen (p) addressed Imam al-Husayn (p): The Prophet Muhammad (p) has ordered you to hand them over (the book and the relics) to this son of yours.
Then, Imam Ali (p) turned towards Imam Zainul Abideen (p): O my son! The Prophet Muhammad (p) has ordered you to hand them over to your son Imam Muhammad al-Baqir (p)…