Mukhtasar Muslim

Abu Huraira (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: Allah, the Mighty and Majestic, laughs at two persons; one killed the other, yet both enter Paradise. The people asked: O Messenger of Allah! How will this happen? He (peace be upon him) said: One fought in the way of Allah and was martyred. Then the one who killed him embraced Islam, fought in the way of Allah, and was martyred.