This refers back to what he had already said (verse 18). Moses lay face down on the ground in the LORD’s presence for forty days.

Because the LORD had said he would destroy you: God threatened to do to the Israelites what he had promised them he would do to the peoples of Canaan.

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .