Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 31
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.