Abu Malik al-Ashja'i narrates from his father that the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight the people until they say 'La ilaha illallah' (there is no god but Allah). So when they say it, they have protected their blood and their wealth from me, except by its right, and their reckoning is with Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8160