John Gill 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 before the Lord forty days and forty nights,
as I fell down at the first
Which Jarchi says are the selfsame said above, (Deuteronomy 9:18) , but doubled or repeated, because of the order of his prayer. The words "at the first" are not in the text; and, as before observed, we do not read that Moses fell down at the first forty days he was in the mount, unless it can be thought he did, (Exodus 32:11) .

Wherefore this falling down seems to be as he fell down at the second forty days; and so this was a third forty days, according to the Jewish writers, and of which opinion were Dr. Lightfoot and others; (See Gill on Exodus 34:28),

because the Lord had said he would destroy you ;
threatened them with destruction, and seemed as if it was his intention to destroy them; nay, even after Moses's first prayer, though he bid him go and lead the people on, yet he declared that he would visit their sin upon them, (Exodus 32:34) .