Sahih Bukhari Volume 9, Book 89, Hadith Number 315.

Narrated By Salama : We gave the oath of allegiance to the Prophet under the tree. He said to me, “O Salama! Will you not give the oath of allegiance?” I replied, “O Allah’s Apostle! I have already given the oath of allegiance for the first time.” He said, (Give it again) for the second time.