Musannaf Ibn Abi Shayba 28:576

Waki’ narrated to us, he said: Sufyan narrated to us, from Habib ibn Abi ‘Amra, from Sa’id ibn Jubayr, who said: Al-Miqdad ibn al-Aswad went out in a military expedition. He said: They passed by a man with some sheep of his, and they wanted to kill him. He said: "There is no god but Allah," but al-Miqdad killed him. It was said to him: "You killed him while he was saying 'There is no god but Allah'?" Al-Miqdad wished that he had escaped with his family and his wealth. He said: When they arrived, they mentioned that to the Prophet, peace and blessings of Allah be upon him, and it was 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,' aspiring for the goods of worldly life." He said: The booty. "For with Allah are many spoils. Thus you were before." He said: You were concealing your faith from the polytheists. "Then Allah conferred His favor upon you." He said: So you manifested Islam. "Investigate." He said: The threat of Allah. "Indeed Allah is ever, with what you do, Acquainted."