Musnad Ahmad 1007:35

'Abd al-Rahman ibn Mahdi narrated to us, al-Aswad ibn Shayban narrated to us, from Khalid ibn Samir, who said: 'Abd Allah ibn Rabah came to us, and I found that a group of people had gathered around him. He said: Abu Qatadah, the horseman of the Messenger of Allah (may Allah bless him and grant him peace), narrated to us, saying: The Messenger of Allah (may Allah bless him and grant him peace) sent the army of the commanders and said: "Upon you is Zayd ibn Harithah. If Zayd is struck down, then Ja'far. If Ja'far is struck down, then 'Abd Allah ibn Rawahah al-Ansari." Ja'far jumped up and said: "May my father and mother be sacrificed for you, O Prophet of Allah! I did not think that you would appoint Zayd over me." He said: "Go forth, for you do not know which of that is better." He said: The army departed and remained for as long as Allah willed. Then the Messenger of Allah (may Allah bless him and grant him peace) ascended the pulpit and ordered that it be called out: "Prayer is a gathering." The Messenger of Allah (may Allah bless him and grant him peace) said: "News has come, or news has arrived" ('Abd al-Rahman was in doubt). "Shall I not inform you about this army of yours that went out to fight? They went forth until they met the enemy. Zayd was struck down as a martyr, so seek forgiveness for him." The people sought forgiveness for him. "Then Ja'far ibn Abi Talib took the banner and charged at the people until he was killed as a martyr. I bear witness to his martyrdom, so seek forgiveness for him. Then 'Abd Allah ibn Rawahah took the banner and planted his feet firmly until he was struck down as a martyr, so seek forgiveness for him." "Then Khalid ibn al-Walid took the banner, and he was not among the commanders, but he appointed himself." The Messenger of Allah (may Allah bless him and grant him peace) raised his two fingers and said: "O Allah, he is a sword from Your swords, so grant him victory." ('Abd al-Rahman said once: "So grant victory through him.") On that day, Khalid was named the Sword of Allah. Then the Prophet (may Allah bless him and grant him peace) said: "Go forth and reinforce your brothers, and let no one remain behind." So the people went forth in intense heat, on foot and as riders.