It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight the people until they say, 'There is no deity worthy of worship except Allah.' So when they say it, they have protected their lives and their wealth from me, except by its right, and their reckoning is with Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5725