Urwah bin al-Zubayr narrated that the Messenger of Allah (peace and blessings be upon him) sent an army to Mu'tah in Jumada al-Ula of the year 8 AH, appointed Zayd bin Harithah (may Allah be pleased with him) as its commander, and said: "If Zayd is killed, then Ja'far bin Abi Talib will be the commander of the people; and if Ja'far is killed, then Abdullah bin Rawahah will be the commander of the people."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4639