John Calvin Commentary Isaiah 16:13

"This is the word that Jehovah spake concerning Moab in time past." — Isaiah 16:13 (ASV)

This is the word. This concluding sentence is the ratification of the prophecy. It means that he has pronounced the decree of God Himself, and that he has brought nothing forward that did not proceed from the Lord; and thus, laying aside his role as a man, he introduces God speaking in this manner.