John Calvin 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. Again, the narrative is blended, for it is certain that this prayer was offered before he remained fasting on the mountain during the second forty days. But since then also, being still in anxiety, he continued the same prayers, it is not surprising that he should include in the forty days’ fast whatever had been done before. For there is no absurdity in supposing that after having obtained the safety of the people, for which he had petitioned, he should still be in trepidation.

Moreover, that this fast was after the prayer which he mentions at the same time may be inferred from the beginning of the next chapter, where he records that the second tablets were given to him, but says not a word about the fast. I have stated why he so often repeats his allusion to the forty days, namely, because it would not have been sufficient merely to intercede unless this reconciliation had followed, which he obtained when he received the new covenant. The rest I have already expounded.