John Gill Commentary Nehemiah 13:3

"And it came to pass, when they had heard the law, that they separated from Israel all the mixed multitude." — Nehemiah 13:3 (ASV)

Now it came to pass, when they had heard the law
Or the law concerning the Ammonite and the Moabite, and which included other nations also, and forbade marriage with them:

that they separated from Israel all the mixed multitude ;
all of these, and other nations, they had contracted affinity with.