Sayyiduna Jarir (may Allah be pleased with him) said: I pledged allegiance to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) set this condition upon me: "That I worship Allah, not associating anything with Him, and act sincerely towards every Muslim."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2304