Sahih Bukhari Volume 4, Book 52, Hadith Number 207.

Narated By Yazid bin Ubaid : Salama said, “I gave the Pledge of allegiance (Al-Ridwan) to Allah’s Apostle and then I moved to the shade of a tree. When the number of people around the Prophet diminished, he said, ‘O Ibn Al-Akwa ! Will you not give to me the pledge of Allegiance?’ I replied, ‘O Allah’s Apostle! I have already given to you the pledge of Allegiance.’ He said, ‘Do it again.’ So I gave the pledge of allegiance for the second time.” I asked ‘O Abu Muslim! For what did you give he pledge of Allegiance on that day?” He replied, “We gave the pledge of Allegiance for death.”