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 get martyred (In Allah's Cause)."
Sahih al-Bukhari Book 56, Hadith 42
← Previous Hadith On the day of the Conquest (of Mecca) the Prophet (ﷺ) said, "There is no emigration after the Conquest but Jihad and intentions. When you are called (by the Muslim ruler) for fighting, go forth immediately." (See Hadith No. 42)

Sahih al-Bukhari Book 56, Hadith 41 Next Hadith → I went to Allah's Messenger (ﷺ) while he was at Khaibar after it had fallen in the Muslims' hands. I said, "O Allah's Messenger (ﷺ)! Give me a share (from the land of Khaibar)."

One of the sons of Sa'id bin Al-'As said, "O Allah's Messenger (ﷺ)! Do not give him a share." I said, "This is the murderer of Ibn Qauqal." The son of Said bin Al-As said, "Strange! A Wabr (i.e. guinea pig) who has come down to us from the mountain of Qaduim (i.e. grazing place of sheep) blames me for killing a Muslim who was given superiority by Allah because of me, and Allah did not disgrace me at his hands (i.e. was not killed as an infidel)." (The sub-narrator said "I do not know whether the Prophet (ﷺ) gave him a share or not.")

Sahih al-Bukhari Book 56, Hadith 43