Charles Ellicott Commentary Deuteronomy 9:25

"So I fell down before Jehovah the forty days and forty nights that I fell down, because Jehovah had said he would destroy you." — Deuteronomy 9:25 (ASV)

Thus I fell down ... —Literally, And I fell down before Jehovah forty days and forty nights, as I had fallen down (originally on the fortieth day) when the Lord said He would destroy you: that is, when He told Moses of the calf.