Rashi on Exodus 34:29/רש"י על שמות ל״ד:כ״ט

1ויהי 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.

2כי 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).