Sayyiduna Jarir (may Allah be pleased with him) said: I pledged allegiance to the Prophet (peace and blessings be upon him) to hear and obey, and he (peace and blessings be upon him) prompted me (to add): "As far as I am able, and to be sincere to every Muslim."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16555