"If a guest enters your home, ask him no quesitons of the Torah unless you know that he can answer! " ~~ Sefer Chassidim
"Who lodges a scholar in his home is as one offering first-fruits [Shavuot] in the Temple. " ~~ Leviticus (Vayikra) R.
"Let a stranger live with you, and he’ll estrange your way of life." ~~ Apocrypha: Ben Sira