Musnad Ahmad 333:18

Hammad ibn Mas'adah narrated to us, from Yazid ibn Abi 'Ubayd, from Salamah ibn al-Akwa', who said: I pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, along with the people at al-Hudaybiyah. Then I sat aside. When the people dispersed from the Messenger of Allah, may Allah bless him and grant him peace, he said: "O son of al-Akwa', will you not pledge allegiance?" I said: I have already pledged allegiance, O Messenger of Allah. He said: "And again." I said: Upon what did you pledge allegiance? He said: "Upon death."