Musnad Ahmad 30:1105

The Messenger of Allah, may Allah bless him and grant him peace, said: "Allah laughs at two men, one of whom kills the other, and both of them enter Paradise." They said: "How, O Messenger of Allah?" He said: "This one is killed and enters Paradise, then Allah turns in repentance to the other, guides him to Islam, then he strives in the way of Allah and is martyred."