Urwah (may Allah have mercy on him) narrates: Then the Messenger of Allah (peace and blessings be upon him) sent an army to Mu'tah and appointed Sayyiduna Zayd bin Harithah (may Allah be pleased with him) as its commander, (and said that) if Zayd was martyred, their commander would be Ja'far, and if Ja'far was martyred, Abdullah bin Rawahah al-Ansari would be their commander. So they set out towards Syria, in the direction of Mu'tah, and the commanders appointed by the Messenger of Allah (peace and blessings be upon him) were martyred. Then Sayyiduna Khalid bin al-Walid (may Allah be pleased with him) took up the banner, and the Muslims agreed upon him, and Allah defeated the enemy.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14027