It is narrated from Sayyiduna Abu Bakr (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever kills a covenant-holder (dhimmi) without due cause, Allah will make Paradise forbidden for him." (That is, the killer will not enter Paradise.)
A "mu'ahid" refers to that individual who, by paying jizyah, resides in the abode of Islam (dar al-Islam); such a person is also called a "dhimmi." The protection of the life and property of such a non-Muslim is the responsibility of the Muslim government. If any Muslim intentionally kills such a person, then his punishment is so severe that Paradise will be forbidden for him, and he will enter Hell.