Sayyiduna Jarir (may Allah be pleased with him) said: I pledged allegiance to the Messenger of Allah (peace and blessings be upon him), and I said: "On hearing and obeying." So he (peace and blessings be upon him) taught me these words: "As far as I am able, and on acting sincerely towards every Muslim."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2341