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