Musnad Ahmad 25:185

Yahya ibn Abi Bukayr narrated to us, 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 Messenger of Allah (peace and blessings of Allah be upon him) while he was driving his sheep. He greeted them with peace, but they said: 'He did not greet us except to seek protection from us.' So they set upon him and killed him, and they brought his sheep to the Prophet (peace and blessings of Allah be upon him)." Then this verse was revealed: {O you who have believed, when you go forth in the cause of Allah, investigate} [An-Nisa': 94].