Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrates: On the day of Khaybar my brother fought fiercely alongside the Messenger of Allah (peace and blessings be upon him); then his sword rebounded on him and killed him. The Companions of the Messenger of Allah (peace and blessings be upon him) spoke about this and had doubts about his affair, (saying) that this man died by his own weapon, and so they fell into doubt about some of his affair. Sayyiduna Salamah (may Allah be pleased with him) says: Then the Messenger of Allah (peace and blessings be upon him) returned from Khaybar, and I said: O Messenger of Allah! Will you permit me to recite rajaz verses before you? The Messenger of Allah (peace and blessings be upon him) gave him permission. Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: Watch what you are saying. So I recited this rajaz: "By Allah! Had it not been for Allah, we would not have been guided, nor would we have given charity, nor prayed." The Messenger of Allah (peace and blessings be upon him) said: "You have spoken the truth." "So send down tranquillity upon us, and make our feet firm if we meet (the enemy). The polytheists have transgressed against us." When I had completed my rajaz, the Messenger of Allah (peace and blessings be upon him) said: "Who said this?" I said: My brother said it. The Messenger of Allah (peace and blessings be upon him) said: "May Allah have mercy on him." I said: O Messenger of Allah! Some people are hesitant to offer the funeral prayer over him, and they say: This man died by his own weapon. The Messenger of Allah (peace and blessings be upon him) said: "He died striving, as a mujahid." Ibn Shihab says: Then I asked the son of Salamah bin al-Akwa', and he narrated to me from his father the same as what Abd al-Rahman had narrated to me, except that Ibn Salamah also said: When I said that they were hesitant to offer the funeral prayer over him, the Messenger of Allah (peace and blessings be upon him) said: "They have lied; he died striving, as a mujahid, and he will have a double reward." And the Messenger of Allah (peace and blessings be upon him) gestured with his two fingers.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6204