Musnad Ahmad 30:2846

Waki' narrated to us, Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, from the Prophet, may God bless him and grant him peace, who said: "God laughs at two men, one of whom kills the other, and both enter Paradise. This one fights in the way of God and is martyred." He said: "Then God turns in repentance to his killer, so he accepts Islam, and he fights in the way of God until he is martyred."