(28) باب الْكَافِرِ يَقْتُلُ الْمُسْلِمَ ثُمَّ يُسْلِمُ فَيُسَدِّدُ بَعْدُ وَيُقْتَلُ
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 whoalso get martyred (In Allah's Cause)."
Reference: Sahih al-Bukhari 2826
In-book reference: Book 56, Hadith 42
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 52, Hadith 80