Targum zu Schemot 20:3

The second word which came forth from the mouth of the Holy One, whose name be blessed, was like storms, and lightnings, and flames of fire. A burning light was on His right hand and on His left and was borne through the air of the heavens, returned, and was made manifest unto the camp of Israel; it returned, and was engraven on the tables of the covenant, and was turned in them from side to side. Then called He, and said, House of Israel, My people, Thou shalt have no other God beside Me.