Exodus 19:11

19:11 and be ready for the third day, for on the third day the Lord will come down on Mount Sinai in the sight of all the people.

Exodus 19:16-20

19:16 On the third day in the morning there was thunder and lightning and a dense cloud on the mountain, and the sound of a very loud horn; all the people who were in the camp trembled. 19:17 Moses brought the people out of the camp to meet God, and they took their place at the foot of the mountain. 19:18 Now Mount Sinai was completely covered with smoke because the Lord had descended on it in fire, and its smoke went up like the smoke of a great furnace, and the whole mountain shook violently. 19:19 When the sound of the horn grew louder and louder, Moses was speaking and God was answering him with a voice.

19:20 The Lord came down on Mount Sinai, on the top of the mountain, and the Lord summoned Moses to the top of the mountain, and Moses went up.