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 therein (compare Exodus 34:9), that is more particularly brought forward here and in Deuteronomy 9:25-29.
Related Commentaries of Deuteronomy 9:18
Deuteronomy 9:18
18 And I fell down before the LORD, as at the first, forty days and forty nights: I did neither eat bread, nor drink water, because of all your sins which ye sinned, in doing wickedly in the sight of the LORD, to provoke him to anger.