Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrated: We set out with the Messenger of Allah (peace and blessings be upon him) for Khaybar, and we were travelling by night. A man among the people said to Amir: O Amir, will you not let us hear some of your verses? And Amir was a poet. So he dismounted and began chanting to drive the camels on for the people, saying (in the rajaz metre): "O Allah, were it not for You, we would not have been guided, nor would we have given charity, nor prayed. So forgive us the shortcomings we have committed, and make our feet firm if we meet (the enemy). Indeed, these people have transgressed against us; when we are called, we come; and with their shouting they have called upon us." Then the Messenger of Allah (peace and blessings be upon him) asked: "Who is this camel-driver?" The people said: Amir bin al-Akwa'. He (peace and blessings be upon him) said: "May Allah have mercy on him." At that a man among the people said: (Martyrdom) has become certain (for him), O Messenger of Allah! If only you had let us benefit from him longer. The narrator said: Then we reached Khaybar and besieged them until we were struck by severe hunger. Then Allah granted us victory over it. On the evening of the day it was conquered, the people were seen with many cooking pots on the fire, so the Messenger of Allah (peace and blessings be upon him) asked: "What are these fires? What are you cooking on them?" The people said: The meat of domestic donkeys. The Messenger of Allah (peace and blessings be upon him) said: "Pour this meat out and break the pots." A man said: O Messenger of Allah, or shall we (just) pour it out? He (peace and blessings be upon him) said: "That is also fine." When the people returned, Amir's sword was rather short. He struck at the shin of a Jew, and the tip of his sword rebounded and struck Amir's knee, and he died from it. I was grieved by this, and when the Messenger of Allah (peace and blessings be upon him) saw my state, he asked: "What is the matter with you?" I said: May my father and mother be ransomed for you, people claim that Amir's deeds have been nullified. The Messenger of Allah (peace and blessings be upon him) said: "They have lied. Who said that? He will have a double reward; he was a striver, a mujahid."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6267