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

Chapter 42
Joy on Friday eve

385. 3. Imam Jafar as-Sadiq (p): Every Friday eve, the Imam experiences a special spiritual pleasure and the Prophet Muhammad (p) and all the Imams (p) go to the Divine Throne, and I am with them. When I return from there, there is an increase in my knowledge.