Musnad Ahmad 30:3691

Muhammad ibn Ubayd narrated to us, Yazid ibn Kaysan narrated to us, from Abu Hazim, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "I have been commanded to fight the people until they say: 'There is no god but Allah.' So if they say: 'There is no god but Allah,' their reckoning is with Allah, the Almighty and Majestic."