Musnad Ahmad 30:3028

Waki' narrated to us, Sufyan narrated to us, from Salih, the freed slave of al-Taw'amah, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him: "I have been commanded to fight the people until they say: 'There is no god but Allah.' If they say: 'There is no god but Allah,' they have protected their blood and their wealth from me, except for its right, and their reckoning is with Allah, the Almighty and Majestic."