(110) باب الْبَيْعَةِ فِي الْحَرْبِ أَنْ لاَ يَفِرُّوا
Narrated Yazid bin Ubaid:
Salama said, "I gave the Pledge of allegiance (Al-Ridwan) to Allah's Messenger (ﷺ) and then I moved to the shade of a tree. When the number of people around the Prophet (ﷺ) diminished, he said, 'O Ibn Al-Akwa` ! Will you not give to me the pledge of Allegiance?' I replied, 'O Allah's Messenger (ﷺ)! 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."
Reference: Sahih al-Bukhari 2960
In-book reference: Book 56, Hadith 171
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 52, Hadith 207