Musnad Abu Ya'la al-Mawsili 195:1

Muhammad ibn Abi Bakr al-Muqaddami narrated to us, Abu 'Awanah narrated to us, from Simak, from al-Nu'man ibn Salim, from Aws, who said: A man came to the Messenger of Allah (ﷺ) and consulted him, or whispered to him, while I was listening. The Messenger of Allah (ﷺ) said: "Go to them and tell them to kill him." He said: Then he called him back, and he returned to him after he had gone. He said to him: "Perhaps he says, 'There is no god but Allah'?" He said: "Yes, by Allah." He said: "Tell them to release him, for it has been revealed to me that I should fight the people until they say, 'There is no god but Allah.' If they say it, their blood becomes forbidden to me, except by right, and their reckoning is with Allah."