Nehemiah 13:1
On that day they read in the book of Moses in the audience of the people; and therein was found written, that the Ammonite and the Moabite should not come into the congregation of God for ever; On that day they read in the book of Moses in the audience of the people; and therein was found written, that the Ammonite and the Moabite should not come into the congregation of God for ever;
The prohibition originated with Moses and stood written in the book of Moses, coming to light when that book was read aloud in the hearing of the people. It barred the Ammonite and the Moabite from ever entering the congregation of God. That perpetual exclusion was God's response to their cruelty and treachery toward Israel.
Tobiah was an Ammonite, a representative of this forbidden race and an enemy of God and His truth. To lodge him in a chamber of the temple that had held the tithes and offerings was to defy this word. Such an act showed contempt for God and profaned His house. Casting out all of Tobiah's household goods from the chamber, cleansing it, and restoring the vessels of the house of God with the grain offering and the frankincense was obedience to this word.