It was narrated that Abu Hurairah said:
"The Messenger of Allah said: 'Allah will laugh at two persons- one of them kills the other, and both of them enter Paradise, for the first one fought in the cause of Allah and was martyred, then his killer repented to Allah and became Muslim, then he also fought in the cause of Allah and was martyred.'"
English reference: Vol. 1, Book 1, Hadith 191
Arabic reference: Book 1, Hadith 196