Sahih Muslim Book 31, Hadith Number 6146.

Chapter : Fraternity established by Allah’s Apostle (may peace be upon him) amongst the companions (Allah be pleased with them).

Jubair b. Mut’im reported Allah’s Messenger (may peace be upon him) as saying: There is no alliance (hilf) in Islam but (the hilf) established in the pre-Islamic days (for good). Islam intensifies and strengthens it.