Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight the people until they say that there is no true deity except Allah. 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 العين / 11439