Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrates: I pledged allegiance to the Messenger of Allah (peace and blessings be upon him) on the day of Hudaybiyah, then moved aside. He (peace and blessings be upon him) said: "O Salamah, will you not pledge allegiance?" I said: I have already pledged allegiance. He (peace and blessings be upon him) said: "Come forward and pledge allegiance." So I came near and pledged allegiance. (Yazid (may Allah have mercy on him) says:) I asked: O Abu Muslim, on what did you pledge allegiance? He said: "Unto death."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6254