Musnad Ahmad 30:3505

Rawh narrated to us, Muhammad ibn Abi Hafsah narrated to us, Ibn Shihab narrated to us, from Sa’id ibn al-Musayyib, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah, the Almighty and Majestic, laughs at two men, one of whom kills the other, and then Allah admits both of them into Paradise." It was said: "How can that be?" He said: "One of them is a disbeliever who kills the other, then he (the killer) embraces Islam, fights in the way of Allah, and is killed."