Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight people until they say 'La ilaha illallah' (there is no god but Allah). When they say 'La ilaha illallah' (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."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / 16503