Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty smiles at two people who enter Paradise even though one of them killed the other." The Companions asked: O Messenger of Allah, how is that possible? He (peace and blessings be upon him) said: "The one who was killed enters Paradise; then Allah grants the killer the ability to accept Islam, and then he is martyred while fighting jihad in the way of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18532
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty smiles at two people who enter Paradise even though one of them killed the other. One was martyred fighting in the way of Allah, while Allah accepted the repentance of the other, and then he too attains martyrdom in the way of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18533