Narrated Ibn 'Umar:

Allah's Messenger (peace be upon him) said: "I have been ordered (by Allah) to fight against the people until they testify that none has the right to be worshiped but Allah and that Muhammad is Allah's Messenger (peace be upon him), and offer the prayers perfectly and give the obligatory charity, so if they perform that, then they save their lives and property from me except for Islamic laws and then their reckoning (accounts) will be done by Allah."

Reference : Sahih-Bukhari 25
In Book Reference : Book 2, Hadith 18