Judgments (Ahkaam)
Bukhari :: Book 9 :: Volume 89 :: Hadith 315
Narrated 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.