Numbers 9:14
And if a stranger shall sojourn among you, and will keep the passover unto the LORD; according to the ordinance of the passover, and according to the manner thereof, so shall he do: ye shall have one ordinance, both for the stranger, and for him that was born in the land.
This ordinance belongs to Israel's first Passover after leaving Egypt, celebrated at Horeb during their stay there lasting nearly or quite a year. A stranger sojourning among Israel was permitted but not compelled to keep the Passover to the Lord. Citizenship in Israel was open to all who chose it, and no one was forced to remain. A stranger who keeps it must do so according to the same ordinance and manner as one born in the land, so that there is one ordinance for both the stranger and the native-born.