Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: "Allah laughs at two men, one of whom kills the other, yet both of them enter Paradise. One of them is a disbeliever who kills the believer and then becomes a Muslim, fights in the way of Allah, and is martyred."