Exodus 34:29

ויהי ברדת משה AND IT CAME TO PASS WHEN MOSES WENT DOWN [FROM MOUNT SINAI] — when he brought the second tablets on the Day of Atonement. כי קרן עור פניו THAT THE SKIN OF HIS FACE BEAMED — קרן is an expression connected with the word קרנים “horns”, and the phrase, קרן אור, the light-“horned” , is used here because light radiates from a point and projects Like a horn. And whence was Moses privileged to have the rays of glory? Our Rabbis said that they originated at the time when he was in the cave, for the Holy One, blessed be He, then put His hand upon his face, as it is said, (Exodus 33:22) “And I will shelter thee with My hand” (Midrash Tanchuma, Ki Tisa 37).

וַיְהִ֗י בְּרֶ֤דֶת מֹשֶׁה֙ מֵהַ֣ר סִינַ֔י וּשְׁנֵ֨י לֻחֹ֤ת הָֽעֵדֻת֙ בְּיַד־מֹשֶׁ֔ה בְּרִדְתּ֖וֹ מִן־הָהָ֑ר וּמֹשֶׁ֣ה לֹֽא־יָדַ֗ע כִּ֥י קָרַ֛ן ע֥וֹר פָּנָ֖יו בְּדַבְּר֥וֹ אִתּֽוֹ׃

And it came to pass, when Moshe came down from mount Sinay with the two tablets of Testimony in Moshe’s hand, when he came down from the mountain, that Moshe knew not that the skin of his face shone while he talked with him.