During the Tabuk campaign of 630 Muhammad sent letters to four towns in the northern Hejaz and Palestine urging them to relinquish maintenance of a military force and rely on Muslims to ensure their security in return for payment of taxes. Moshe Gil argues that these texts represent the paradigm of letters of security that would be issued by Muslim leaders during the subsequent early conquests, including the use of the word jizya, which would later take on the meaning of poll tax.