I saw these words last September at the Shakespeare and Company Bookstore in Paris. George Whitman, the long time owner of the bookstore hand-painted the words on the wall himself. They are a paraphrase of a passage found in the Christian scriptures...
"Do
not forget to show hospitality to strangers, for by so doing some
people have shown hospitality to angels without knowing it."
- Hebrews 13:2
I made a small sign with these words and put them on the door to my classroom.