Moreover (1): the life-blood of a Muslim may be shed only for one of three reasons—apostasy, adultery committed by someone previously married, or deliberate homicide. For that very reason the Companions, and the Muslims after them, regarded those who withheld zakah as apostates; they did not consider a single one of them to be Muslim. Thus, anyone who refused to pay zakah and was put to death for it, having still failed to discharge it, was nothing but an unbeliever. The same judgment applies to fasting and to the pilgrimage: were it imagined that a person were told, “If you do not fast we will kill you,” and he persisted in abstaining from both fasting and hajj until he was executed, he would be an unbeliever as well. These obligations upon which Islam is built stand on the same footing as the two testimonies of faith; without them one cannot be a Muslim.
In the Abode of Islam no one is allowed to remain except a Muslim or a non-Muslim who pays the jizyah in a state of subjugation. Therefore, if someone does not fall under the category of a non-Muslim paying the jizyah in such subservience, he is deemed a Muslim, and he is not to be counted among the Muslims