Sahih Muslim 1802 b
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 God, if God 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 God show mercy to him! I said: By God, some people are reluctant to invoke God'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 God'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 God's blessings on him, the Messenger of Allah (may peace be, upon him said: They lied. ('Amir) died as God'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.
وَحَدَّثَنِي أَبُو الطَّاهِرِ، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، أَخْبَرَنِي عَبْدُ الرَّحْمَنِ، - وَنَسَبَهُ غَيْرُ ابْنِ وَهْبٍ فَقَالَ ابْنُ عَبْدِ اللَّهِ بْنِ كَعْبِ بْنِ مَالِكٍ - أَنَّ سَلَمَةَ، بْنَ الأَكْوَعِ قَالَ لَمَّا كَانَ يَوْمُ خَيْبَرَ قَاتَلَ أَخِي قِتَالاً شَدِيدًا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَارْتَدَّ عَلَيْهِ سَيْفُهُ فَقَتَلَهُ فَقَالَ أَصْحَابُ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي ذَلِكَ وَشَكُّوا فِيهِ رَجُلٌ مَاتَ فِي سِلاَحِهِ ‏.‏ وَشَكُّوا فِي بَعْضِ أَمْرِهِ ‏.‏ قَالَ سَلَمَةُ فَقَفَلَ رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْ خَيْبَرَ فَقُلْتُ يَا رَسُولَ اللَّهِ ائْذَنْ لِي أَنْ أَرْجُزَ لَكَ ‏.‏ فَأَذِنَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالَ عُمَرُ بْنُ الْخَطَّابِ أَعْلَمُ مَا تَقُولُ قَالَ فَقُلْتُ وَاللَّهِ لَوْلاَ اللَّهُ مَا اهْتَدَيْنَا وَلاَ تَصَدَّقْنَا وَلاَ صَلَّيْنَا فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ صَدَقْتَ ‏"‏ ‏.‏ وَأَنْزِلَنَّ سَكِينَةً عَلَيْنَا وَثَبِّتِ الأَقْدَامَ إِنْ لاَقَيْنَا وَالْمُشْرِكُونَ قَدْ بَغَوْا عَلَيْنَا قَالَ فَلَمَّا قَضَيْتُ رَجَزِي قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ مَنْ قَالَ هَذَا ‏"‏ ‏.‏ قُلْتُ قَالَهُ أَخِي فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ يَرْحَمُهُ اللَّهُ ‏"‏ ‏.‏ قَالَ فَقُلْتُ يَا رَسُولَ اللَّهِ إِنَّ نَاسًا لَيَهَابُونَ الصَّلاَةَ عَلَيْهِ يَقُولُونَ رَجُلٌ مَاتَ بِسِلاَحِهِ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ مَاتَ جَاهِدًا مُجَاهِدًا ‏"‏ ‏.‏ قَالَ ابْنُ شِهَابٍ ثُمَّ سَأَلْتُ ابْنًا لِسَلَمَةَ بْنِ الأَكْوَعِ فَحَدَّثَنِي عَنْ أَبِيهِ مِثْلَ ذَلِكَ غَيْرَ أَنَّهُ قَالَ - حِينَ قُلْتُ إِنَّ نَاسًا يَهَابُونَ الصَّلاَةَ عَلَيْهِ - فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ كَذَبُوا مَاتَ جَاهِدًا مُجَاهِدًا فَلَهُ أَجْرُهُ مَرَّتَيْنِ ‏"‏ ‏.‏ وَأَشَارَ بِإِصْبَعَيْهِ ‏.‏