Jami` at-Tirmidhi 3030

Narrated Ibn 'Abbas:"A man from Banu Sulaim who had some sheep with him, passed by some of the Companions of the Messenger of Allah (ﷺ). He gave Salam to them and they said: 'He did not give Salam except to protect himself.' So they attacked him, killed him, and took his sheep. They went to the Messenger of Allah (ﷺ) with them, and Allah, Most High, revealed: O you who believe! When you go in the cause of Allah, verify and say not to anyone who greets you: "You are not a believer (4:)