From Sahih Bukhari (Hadith 2826):
Narrated Abu Huraira: Allah's Messenger (ﷺ) 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 gets martyred (in Allah's Cause)
Reference: Bukhari 2826