Khalid bin Sameer says that once Abdullah bin Rabah came to us. I saw that many people had gathered around him and they were saying that Abu Qatadah (may Allah be pleased with him) told us that once the Prophet Muhammad (peace and blessings be upon him) dispatched an army called "Jaish Umara" and said: "Your commander is Zaid bin Harithah. If Zaid is martyred, then Ja'far will be the commander. If Ja'far is also martyred, then Abdullah bin Rawahah Al-Ansari will be the commander." Upon this, Ja'far (may Allah be pleased with him) said, "O Messenger of Allah! May my parents be sacrificed for you, I did not think that you would appoint Zaid as commander over me." The Prophet (peace and blessings be upon him) said, "Set out, for you do not know in which matter there is goodness for you." So that army set out. After some time had passed, one day the Prophet (peace and blessings be upon him) ascended the pulpit and ordered the call for "the prayer is ready" to be made and said, "There is a distressing news. Shall I not inform you about that army of Mujahideen?" They set out from here and when they faced the enemy, Zaid was martyred. Pray for his forgiveness." The people did so. Then Ja'far bin Abi Talib took the flag and attacked the enemy fiercely until he too was martyred. I bear witness to his martyrdom, so pray for his forgiveness as well. Then Abdullah bin Rawahah took the flag and stood firm with great perseverance until he too was martyred. Pray for his forgiveness as well. Then Khalid bin Walid took the flag, even though no one had appointed him as commander. Then the Prophet (peace and blessings be upon him) raised his finger and said, "O Allah! He is one of Your swords, so help him." From that day, Khalid bin Walid (may Allah be pleased with him) was called "Saifullah" (the Sword of Allah). Then the Prophet (peace and blessings be upon him) said, "Set out to help your brothers, and let no man stay behind." So, in that intense heat, people set out on foot and mounted.