Exodus 34:28

And he was there before the Lord forty days and forty nights; he ate no bread nor did he drink water; and he wrote upon the other tables the words of the covenant, the Ten Words which had been written upon the former tables.

AND HE WAS THERE WITH THE ETERNAL FORTY DAYS AND FORTY NIGHTS… AND HE WROTE UPON THE TABLETS THE WORDS OF THE COVENANT, THE TEN WORDS. The verse is stating that Moses was on top of the mountain for forty days and forty nights, and he wrote there upon the Tablets. And in the opinion of our Rabbis the meaning of the verse is that Moses was there with G-d for forty days and forty nights, during which time He wrote on the Tablets the words of this second covenant. But he had stayed there for another forty days prior to these to pray for all his requests, as is narrated in the section beginning, See, Thou sayest unto me: Bring up this people; and with reference to these days it is said in the Book of Deuteronomy, So I fell down before the Eternal the forty days and forty nights that I fell down; because the Eternal had said He would destroy you. Here, however, Scripture did not explain how many days were the duration of this period of supplication and prayer, for it is known that it was from the time he went up to the mountain until he came down to hew the second Tablets. But it said that the matter of the second Tablets was like that of the first in every respect — the writing of G-d, and the number of days he stood before G-d, so that he should not think that he had already learned the Torah during the first time, and that therefore it would not be necessary for him to stay there many days.

‘Va’yichtov’ upon the Tablets — this means that G-d wrote, and does not refer back to Moses [mentioned in the preceding phrase, he did neither eat bread, nor drink water], for so He said, and I will write upon the Tablets. Similarly, in the Book of Deuteronomy it says, and I hewed two Tablets of stone like unto the first, and went up into the mount, having the two Tablets in my hand. And He wrote on the Tablets, according to the first writing. And since it says according to the first writing, we know that they were written with the finger of G-d. Thus the meaning of the expression, and I will write means “with the finger,” and you know [already] the meaning of “finger” from that of “the hand.”, 14:31. See Ramban there.

ויכתוב על הלוחות, G’d Himself inscribed the Tablets. The same words as had been inscribed on the first Tablets were inscribed on the second set. (as per verse 1 of our chapter) The same information is repeated by Moses in Deuteronomy 10,4

'ויהי שם עם ה, his third and final 40 day stay there..