Exodus 19:16-19

When morning dawned on the third day, there was thunder, lightning, and a thick cloud on the mountain, and a very loud blast of a horn. All the people in the camp shook with fear. Moses brought the people out of the camp to meet God, and they took their place at the foot of the mountain. Mount Sinai was all in smoke because the LORD had come down on it with lightning. The smoke went up like the smoke of a hot furnace, while the whole mountain shook violently. The blasts of the horn grew louder and louder. Moses would speak, and God would answer him with thunder.