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: 6098
Jabir b. Abdullah reported that it was concerning them (the Ansar) that this verse was revealed, that when the two groups amongst you were about to lose heart and Allah was the Guardian of them both. This concerned Banu Salama and Banu Haritha and we did not like that Allah, the Exalted and Glorious, should not have revealed this verse for the fact that Allah (gave an assurance) of being the Guardian of both.