Exodus 19:9

9And the Lord said to Moses, “Behold, I am coming to you ain a thick cloud, that bthe people may hear when I speak with you, and may also cbelieve you forever.”

When Moses told the words of the people to the Lord,

Exodus 19:16

16On the morning of the dthird day there were ethunders and lightnings and fa thick cloud on the mountain and a very loud gtrumpet blast, so that all the people in the camp htrembled.
Copyright information for ESV