Albert Barnes 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)

I fell down before the Lord, as at the first — Moses interceded for the people before he came down from the mountain the first time (Exodus 32:11–13). This intercession is only briefly alluded to in this verse. Afterward, he spent another 40 days on the mountain in fasting and prayer to obtain a complete restitution of the covenant (Exodus 34:28). It is this second forty days, and the intercession of Moses made during that time , that is more particularly brought forward here and in Deuteronomy 9:25-29.