Musnad Ahmad 30:208

Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, attributing it to the Prophet (may Allah bless him and grant him peace): "Indeed, Allah the Almighty laughs at two men, one of whom killed the other, yet both enter Paradise." He says: "One was a disbeliever who killed a Muslim, then the disbeliever accepted Islam before he died, so Allah the Almighty admitted them both into Paradise."