Deutéronome 9:25

And I bowed down in prayer before the Lord for the forty days and nights in which I was prostrate in supplication, because the Lord had said He was about to destroy you.

AND I FELL DOWN BEFORE THE ETERNAL. He is stating: “I had to cast myself down in prayer for you forty days and forty nights because of this great sin, for at first G-d said to destroy you, until I prayed to G-d and said ‘[O G-d Eternal], destroy not Thy people and Thine inheritance.’” This was his prayer before he came down from the mountain, and it was because of this prayer that he was answered with And the Eternal repented of the evil which He said He would do unto His people., Verse 14. The word “repented” here is an anthropomorphism (Ibn Ezra). He did not mention the prayer of the forty days and forty nights, for the measure thereof is longer than the earth, and broader than the sea. I have already explained. the reason why the Divine Names are written both with Aleph Daleth, and Yod Hei [in Verse 26: O G-d Eternal, destroy not Thy people etc.].

'ואתנפל לפני ה' את ארבעים היום וגו' ואתפלל אל ה ואומר וגו', who is wise enough to understand and explain why Moses had to repeat that he had to fall on his face for forty days? Is it then the custom for the Torah to keep repeating things like ואתנפל אשר התנפלתי?. We would have expected the words ואתפלל, “I prayed,” to immediately follow the words ואתנפל, “I prostrated myself! However, there is a profound wisdom in the sequence of the words the Torah quotes Moses as saying, one that contains an admonition to his people. Moses is using the way he phrases his report of what he did during those 40 days and how he did it, by pre-empting an argument by the Jewish people that just as his prayers had been helpful when they were in the desert, similar prayers would be equally helpful once the people were in the Holy Land and they committed a major sin. The prophets in such days, they would argue, would surely be able to act as their interlocutors then just as Moses had done while he was alive. The point Moses is making here is that at this time their atonement was only in order for G’d not to be perceived as being unable to fulfill His oath of bringing Avraham’s descendants to take over the land of Canaan. In other words, Israel’s pardon was for the sake of not desecrating G’d’s Holy name. Once G’d had demonstrated that He had kept His promise, and that the people concerned had been settled in the Holy Land, as promised, His Holy name would never again be in danger of being desecrated by someone citing the arguments Moses had been able to cite while the people were still in the desert. This is why Moses repeats again the wording of his prayer. He is telling the people that if his prayer had been heard, it was only on account of an irrefutable argument he had been able to advance, namely how the Egyptians and other nations would react to the Jewish people’s annihilation. It follows, that if, G’d forbid the Jewish people, once settled in the Holy Land, would become guilty of similar misdemeanours as had the Canaanites, they would face both death and expulsion. On the contrary, if that were to happen, the gentile nations would not point to G’d’s inability to keep them there as the cause of their misfortune, but they would correctly point to the disloyalty this people had demonstrated by eschewing the laws given to them by their G’d, the One who had driven out the Canaanites. We find this all spelled out in Deuteronomy 29,23-27.