Albert Barnes Commentary 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 an Ammonite and a Moabite should not enter into the assembly of God for ever," — Nehemiah 13:1 (ASV)

On that day - Or, “at that time,” as in (Nehemiah 12:44).

The entire Pentateuch is probably meant by “the Book of Moses”.