Rashi on Ki Sisa 34:33

English: ויתן על פניו מסוה AND HE PUT A VEIL ON HIS FACE — Render it (מסוה) as the Targum does: בית אפי, a cover for his face. מסוה is an Aramaic expression (from the root סוי, “to look”). It occurs in the Talmud (Ketubot 62b) “her heart perceived (סּוי), and again in Ketubot 60a “הוה קא מסוה לאפה”, where מסוה is an expression for looking: “he looked into her face”, i. e. he gazed at her. Here, too, מסוה denotes a cloth that was put in front of the face and of the region of the eyes. And out of reverence for the “rays of glory” — that not everybody should feast himself on them — Moshe used to put the veil in front of them (the eyes), but took it off during the time when he spoke to Israel, and during