Midrasch zu Schemot 19:21

(The Holy One, blessed be He, said:) Let Moses descend to the camp, and afterwards will I cause My Torah to be proclaimed. He said to him: "Go down, charge the people" (Ex. 19:21). Moses was wishing to be || there, and he said to Him: I have already charged the people. He said to him: Go, and call thy Rabbi. Moses descended to the camp to call Aaron, and the Holy One, blessed be He, proclaimed His Torah unto His people, as it is said, "So Moses went down unto the people, and told them" (Ex. 19:25). What is written after this? "And God spake all these words, saying, I, the Lord, am (to be) thy God, who brought thee out of the land of Egypt, out of the house of bondage" (Ex. 20:1, 2).