Sahih Muslim Book 1, Hadith Number 33.

Chapter : Command for fighting against the people so long as they do not profess that there is no god but Allah and Muhammad is His Messenger.

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.