Sahih Bukhari Volume 001, Book 002, Hadith Number 055.
Narated By Ziyad bin'Ilaqa : I heard Jarir bin 'Abdullah (Praising Allah). On the day when Al-Mughira bin Shu'ba died, he (Jarir) got up (on the pulpit) and thanked and praised Allah and said, "Be afraid of Allah alone Who has none along with Him to be worshipped.(You should) be calm and quiet till the (new) chief comes to you and he will come to you soon. Ask Allah's forgiveness for your (late) chief because he himself loved to forgive others." Jarir added, "Amma badu (now then), I went to the Prophet and said, 'I give my pledge of allegiance to you for Islam." The Prophet conditioned (my pledge) for me to be sincere and true to every Muslim so I gave my pledge to him for this. By the Lord of this mosque! I am sincere and true to you (Muslims). Then Jarir asked for Allah's forgiveness and came down (from the pulpit).