Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight until they say 'La ilaha illallah' (There is no god but Allah). Whoever says the testimony has protected his wealth and his life from us, except for the rights of Islam, and his reckoning is with Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17947