From Sahih Muslim (Hadith 22):
It has been narrated on the authority of Abdullah b. 'Umar that the Messenger of Allah said: I have been commanded to fight against people till they testify that there is no god but Allah, that Muhammad is the messenger of Allah, and they establish prayer, and pay Zakat and if they do it, their blood and property are guaranteed protection on my behalf except when justified by law, and their affairs rest with Allah.
Reference: Muslim 22