The Messenger of Allah (ﷺ) sent an army to Mu'tah and he appointed Zaid in charge of it.

If Zaid was killed, Ja'far [was to assume command]. If Ja'far was killed, Ibn Rawahah [was to assume command]. Ibn Rawahah stayed behind and prayed Jumu'ah with the Messenger of Allah (ﷺ) who saw him and said, “What kept you behind?” He said: I wanted to pray Jumu’ah with you. He said: "To go out in the morning (to fight, for the sake of Allah) and come back in the evening is better than this world and everything in it.”