Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this chain (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 (in a manner befitting His Majesty), one of whom kills the other, yet both enter Paradise. One of them fights in the way of Allah and is killed, then Allah grants the killer the ability to repent (accept Islam), then he fights and is martyred."