From Sahih Bukhari (Hadith 4261):
Abdullah bin `Umar said:"Allah's Messenger (ﷺ) appointed Zaid bin Haritha as the commander of the army during the Ghazwa of Mu'tah and said, "If Zaid is martyred, Ja`far should take over his position, and if Ja`far is martyred, `Abdullah bin Rawaha should take over his position.' " `Abdullah bin `Umar further said, "I was present amongst them in that battle and we searched for Ja`far bin Abi Talib and found his body amongst the bodies of the martyred ones, and found over ninety wounds over his body, caused by stabs or shots (of arrows)
Reference: Bukhari 4261