In Behar, second last verse 26:1 "...you shall not place a pavement stone on which to prostrate yourselves..." Rashi says, "...the Torah prohibits one to [prostrate yourselves] outside the Holy Temple [where prostration on a stone floor is permitted]..." I cannot find where this is written in Torah. Does Torah really say that, and if so, where?

The verse you quoted is the source of the prohibition. It specifically says that one should not make a stone floor for the purpose of bowing, and juxtaposes this with the prohibition of idolatry.