Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) says: On the day of Khaybar my brother fought very bravely. His own sword struck him and he was martyred. The Companions (may Allah be pleased with them) told him (peace and blessings be upon him) that this man had been killed by his own weapon, so he (peace and blessings be upon him) said: "He died striving, a mujahid, (and is a martyr)."
Ibn Shihab (may Allah have mercy on him) says: I asked the sons of Salamah bin al-Akwa', and they narrated the same from their father, and they also mentioned that the Prophet (peace and blessings be upon him) had said: "They have spoken falsely. He died striving, a mujahid; for him is a double reward."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16392