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 of Allah be upon him) said: "Allah laughs at two men, one of whom kills the other, yet both enter Paradise. (The situation is like this:) One man fights in the way of Allah and is martyred, then Allah grants the killer the ability to repent, so he accepts Islam and then fights in the way of Allah and is martyred."