Exodus 19:9; Exodus 19:18

9 The LORD said to Moses, “I am going to come to you in a dense cloud, so that the people will hear me speaking with you and will always put their trust in you.” Then Moses told the LORD what the people had said.

18 Mount Sinai was covered with smoke, because the LORD descended on it in fire. The smoke billowed up from it like smoke from a furnace, and the whole mountain trembled violently.