Sahih Muslim Book 1, Hadith Number 103.

Chapter : Din is sincerity and well-wishing.

It is narrated on the authority of Jarir that he observed: I owed allegiance to the Apostle of Allah (may peace and blessings be upon him) on hearing (is commands) and obeying (them) and the Prophet) instructed me (to act) as lay in my power, and sincerity and goodwill for every Muslim.