Exodus 34:27-29

Then the LORD said to Moses, “Write down these words, for in accordance with these words I have made a covenant with you and with Israel.” So he was there with the LORD forty days and forty nights; he did not eat bread or drink water. And he wrote on the tablets the words of the covenant, the Ten Commandments. It came about when Moses was coming down from Mount Sinai (and the two tablets of the testimony were in Moses’ hand as he was coming down from the mountain), that Moses did not know that the skin of his face shone because of his speaking with Him.