Musnad Ahmad 30:3124

Surayj ibn al-Nu‘man narrated to us, Fulayh narrated to us, from Hilal, from ‘Abd al-Rahman, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I will continue 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 wealth and their lives from me, except by its right, and their reckoning is with Allah, the Almighty and Majestic."