English Translation
Narrated Abu Huraira: Allah's Apostle said, "Allah welcomes two men with a smile; one of whom kills the other and both of them enter Paradise. One fights in Allah's Cause and gets killed. Later on Allah forgives the 'killer who also get martyred (In Allah's Cause)."
This hadith is from Sahih al-Bukhari, the most rigorously authenticated collection in Islamic scholarship. No editorial opinion is added.