Sahih Bukhari Volume 8, Book 73, Hadith Number 169.
The infidels have made a hue and cry to ask others help against us. Allah’s Apostle said, “Who is that driver (of the camels)?” They said, “He is ‘Amir bin Al-Aqwa.”‘ He said, “May Allah bestow His mercy on him.” A man among the people said, Has Martyrdom been granted to him, O Allah’s Prophet! Would that you let us enjoy his company longer.” We reached (the people of) Khaibar and besieged them till we were stricken with severe hunger but Allah helped the Muslims conquer Khaibar. In the evening of its conquest the people made many fires. Allah’s Apostle asked, “What are those fires? For what are you making fires?” They said, “For cooking meat.” He asked, “What kind of meat?” They said, “Donkeys’ meat.” Allah’s Apostle said, “Throw away the meat and break the cooking pots.” A man said, O Allah’s Apostle! Shall we throw away the meat and wash the cooking pots?” He said, “You can do that too.” When the army files aligned in rows (for the battle), ‘Amir’s sword was a short one, and while attacking a Jew with it in order to hit him, the sharp edge of the sword turned back and hit ‘Amir’s knee and caused him to die.
When the Muslims returned (from the battle), Salama said, Allah’s Apostle saw me pale and said, ‘What is wrong with you?”‘ I said, “Let my parents be sacrificed for you! The people claim that all the deeds of Amir have been annulled.” The Prophet asked, “Who said so?” I replied, “So-and-so and so-and-so and Usaid bin Al-Hudair Al-Ansari said, ‘Whoever says so is telling a lie. Verily, ‘Amir will have double reward.”‘ (While speaking) the Prophet put two of his fingers together to indicate that, and added, “He was really a hard-working man and a Mujahid (devout fighter in Allah’s Cause) and rarely have there lived in it (i.e., Medina or the battle-field) an “Arab like him.”