Musannaf Ibn Abi Shayba 36:431

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Hajjaj, from al-Hakam, from Miqsam, from Ibn Abbas, that the Messenger of Allah (may Allah bless him and grant him peace) sent [an army] to Mu'tah and appointed Zayd in command. [He said]: "If Zayd is killed, then Ja'far; and if Ja'far is killed, then Ibn Rawahah." Ibn Rawahah stayed behind to pray with the Prophet (may Allah bless him and grant him peace). The Prophet (may Allah bless him and grant him peace) saw him and said: "What kept you behind?" He said: "To pray with you." He said: "A morning or an evening in the path of Allah is better than the world and what is in it."