This reading is from the special Torah portion read on the Shabbat during Passover, Parshat Pesach, Exodus 33:12-34:26.
"But," He said, "you cannot see My face, for man may not see Me and live." And the Lord said, "See, there is a place near Me. Station yourself on the rock and, as My Presence passes by, I will put you in a cleft of the rock and shield you with My hand until I have passed by. Then I will take My hand away and you will see My back; but My face must not be seen."