Sforno on Exodus 34:33/ספורנו על שמות ל״ד:ל״ג

1ויכל משה מדבר אתם, but while he was still speaking to them he did not wear the cloth. This corresponds to Isaiah 30,20 “your eyes should look at your teachers.” As our sages explained (Eyruvin 13, Rabbi Yehudah Hanassi speaking) “if I had looked at his (Rabbi Meir’s) teacher’s face instead of merely at his back I would have been even more learned.”)