Musnad Ahmad 25:604

Husayn ibn Muhammad and Khalaf ibn al-Walid narrated to us, they said: Isra’il narrated to us, from Simak, from ‘Ikrimah, from Ibn ‘Abbas, who said: A man from Banu Sulaym passed by a group of the companions of the Prophet, may Allah bless him and grant him peace, while he was driving his sheep. He greeted them with peace, so they said: "He did not greet you with peace except to seek protection from you." They went to him and killed him, and they took his sheep, and they brought them to the Prophet, may Allah bless him and grant him peace. Then Allah, the Almighty and Majestic, revealed: "O you who have believed, when you go forth in the cause of Allah, investigate; and do not say to one who gives you [a greeting of] peace, 'You are not a believer,'" [An-Nisa: 94] to the end of the verse.