Exodus 19:9; Exodus 19:16

9 And the LORD said to Moses, "Behold, I am coming to you in a thick cloud, that the people may hear when I speak with you, and may also believe you forever."When Moses told the words of the people to the LORD,

16 On the morning of the third day there were thunders and lightnings and a thick cloud on the mountain and a very loud trumpet blast, so that all the people in the camp trembled.