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