Musnad Ahmad 30:3387

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have been commanded to fight the people until they say: 'There is no god but Allah.' If they say it, their blood and their wealth are protected from me, except by its right, and their reckoning is with Allah."