It has been reported on the authority of Salama b. Akwa' who said

On the day of the Battle of Khaibar my brother fought a fierce fight by the side of the Messenger of Allah (ﷺ). His sword rebounded and killed him. The Companions of the Messenger of Allah (may peace be upon hill) talked about his death and doubted (whether it was martyrdom). (They said): (He is) a man killed by his own weapon, and expressed doubt about his affair. Salama said: When the Messenger of Allah (ﷺ) returned from Khaibar, I said: Messenger of Allah, permit me that I may recite to you some rajaz verses. The Messenger of Allah (ﷺ) permitted him. 'Umar b. Khattab said: I know what you will recite. I recited: By Allah, if Allah had guided us not, We would hive neither been guided aright nor practised charity, Nor offered prayers. The Messenger of Allah (ﷺ) said: What you have said is true, 'I (continued): And descend on us peace and tranquillity And keep us steadfast if we encounter (with our enemies) And the polytheists have rebelled against us. When I finished my rajaz, the Messenger of Allah (ﷺ) said: Who composed these verses? I said: They were composed by my brother. The Messenger of Allah (ﷺ) said: May Allah show mercy to him! I said: By Allah, some people are reluctant to invoke Allah's mercy on him (because) they say he is a man who died by his own sword. (Hearing this) the Messenger of Allah (ﷺ) said: He died as Allah's devotee and warrior. Ibn Shihab has said: I asked one of the sons of Salama (b. Akwa') about (the death of 'Amir). He related to me a similar tradition except that he said: When I said some people were reluctant invoke Allah's blessings on him, the Messenger of Allah (may peace be, upon him said: They lied. ('Amir) died as Allah's devotee and warrior (in the cause of Allah). For him there is a double reward, and he pointed out this by putting his two fingers together.