Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 6074
Al-Bari' b. 'Azib reported: I heard Allah's Messenger (may peace be upon him) as saying: Hassan b. Thibit, write satire (against the non-believers) ; Gabriel is with you. This hadith has been narrated on the authority of Shu'ba with the same chain of transmitters.