Musnad Ahmad 1007:50

'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 al-Ansari came to us, and the Ansar used to seek his legal understanding. I came to him while he was in the enclosure of Sharik ibn al-A'war, which opens onto the al-Mirbad, and a group of people had gathered around him. He said: Abu Qatada al-Ansari, 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 an army of commanders and said: "Upon you is Zayd ibn Harithah. If Zayd is struck down, then Ja'far ibn Abi Talib. 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 Messenger of Allah! I did not fear that you would appoint Zayd over me." He said: "Go forth, for you do not know which of that is better." They 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 announced: "Prayer is a gathering." The Messenger of Allah (may Allah bless him and grant him peace) said: "News has come, or news has arrived, or news has returned" ('Abd al-Rahman was uncertain). "Shall I not inform you about this army of yours that went out? They went forth and met the enemy, and Zayd was struck down as a martyr, so ask forgiveness for him." The people asked 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 ask forgiveness for him. Then 'Abd Allah ibn Rawahah took the banner and planted his feet firmly until he was killed as a martyr, so ask forgiveness for him. Then Khalid ibn al-Walid took the banner, and he was not among the commanders, but he appointed himself." Then 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." From that day on, Khalid was called the Sword of Allah. Then he said: "Go forth and reinforce your brothers, and let no one remain behind." He said: So the people went forth in intense heat, on foot and as riders.