Sayyiduna Jarir (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) to give him the pledge of allegiance, and he (peace and blessings be upon him) said: "O Jarir, stretch out your hand." He asked: On what? He (peace and blessings be upon him) said: "On condition that you surrender yourself to Allah, and that you wish well for every Muslim." Sayyiduna Jarir (may Allah be pleased with him) understood this, and he was an intelligent man, so he said: O Messenger of Allah, as far as I am able. So this became a concession for people after him.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2352