From Sahih Bukhari (Hadith 4169):
Narrated Yazid bin Abi Ubaid:I said to Salama bin Al-Akwa`, "For what did you give the Pledge of allegiance to Allah's Messenger (ﷺ) on the day of Al-Hudaibiya?" He replied, "For death (in the Cause of Islam)
Reference: Bukhari 4169