Charles Ellicott Commentary Deuteronomy 9:18

"And I fell down before Jehovah, as at the first, forty days and forty nights; I did neither eat bread nor drink water; because of all your sin which ye sinned, in doing that which was evil in the sight of Jehovah, to provoke him to anger." — Deuteronomy 9:18 (ASV)

And I fell down before the Lord, as at the first, forty days and forty nights. —Moses had already interceded for them in Sinai before he came down on the fortieth day (Exodus 32:11–14). He now spent forty days and nights in the work of intercession.

We are not to understand that the first forty days were spent in this way. At that time he received the pattern of the tabernacle and the directions for the priesthood, which he did not deliver to Israel until after he descended from Sinai the second time (see Exodus 24:18 and Exodus 35:1 and following). During the first forty days, Joshua was with Moses on the mountain (probably to help in taking the pattern for the tabernacle); during the second forty, Moses was alone.