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: 6100
Anas reported that Allah's Messenger (may peace be upon him) sought forgiveness for the Ansar and he said: I think (he also sought forgiveness) for the children of the Ansar and the slaves and the freed men of the Ansar. I have no doubt about it.