Rambam Laws of Avoda Zara Chapter 10

The G-d of Israel commanded us regarding non Jews "Lo Yeshvu BeArtzecha" - "They shall NOT live in your land" Exodus 23/33 
Rambam Laws of Avoda Zara Chapter 10, Halacha 6....When, however, Israel is in power over them, it is forbidden for us to allow a non Jew involved in foreign worship among us. Even a temporary resident or a merchant who travels from place to place should not be allowed to pass through our land until he accepts the seven universal laws commanded to Noah and his descendants, as [Exodus 23:33] states: 

"They shall not dwell in your land" - i.e., even temporarily.
A person who accepts these seven mitzvot is a ger toshav. A ger toshav may be accepted only in the era when the [laws of the] Jubilee Year are observed. In an era when the [laws of the] Jubilee Year are not observed, however, we may accept only full converts [to Judaism].