Musnad Ahmad 30:1783

Aswad ibn Amir narrated to us, Abu Bakr informed us, from Asim, from Abu Salih, 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 for a right of justice, and their reckoning is with Allah."