It is reported on the authority of Abu Huraira that he heard the Messenger of Allah say: I have been commanded to fight against people, till they testify to the fact that there is no god but Allah, and believe in me (that) I am the messenger (from the Lord) and in all that I have brought. And when they do it, their blood and riches are guaranteed protection on my behalf except where it is justified by law, and their affairs rest with Allah.

Sahih Muslim 1:31 - Sunni Hadith