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-Fada'il Al-Sahabah)
Muslim :: Book 31 : Hadith 5938
Jabir b. Abdullah reported: I heard Allah's Messenger (may peace be upon him) exhorting people on the Day of the Battle of the Ditch to fight. Zubair said: I am ready (to participate). He then again exhorted and he again said: I am ready to participate. Thereupon Allah's Messenger (may peace be upon him) said: Behold. for every Prophet there is a helper and my helper is Zubair.