Exodus 32:15

32:15 Moses turned and went down from the mountain with the two tablets of the testimony in his hands. The tablets were written on both sides – they were written on the front and on the back.

Exodus 34:4

34:4 So Moses cut out two tablets of stone like the first; early in the morning he went up to Mount Sinai, just as the Lord had commanded him, and he took in his hand the two tablets of stone.