Exodus 19:1-9, 16

On exactly the third-month anniversary of the Israelites’ leaving the land of Egypt, they came into the Sinai desert. They traveled from Rephidim, came into the Sinai desert, and set up camp there. Israel camped there in front of the mountain while Moses went up to God. The LORD called to him from the mountain, “This is what you should say to Jacob’s household and declare to the Israelites: You saw what I did to the Egyptians, and how I lifted you up on eagles’ wings and brought you to me. So now, if you faithfully obey me and stay true to my covenant, you will be my most precious possession out of all the peoples, since the whole earth belongs to me. You will be a kingdom of priests for me and a holy nation. These are the words you should say to the Israelites.” So Moses came down, called together the people’s elders, and set before them all these words that the LORD had commanded him. The people all responded with one voice: “Everything that the LORD has said we will do.” Moses reported to the LORD what the people said. Then the LORD said to Moses, “I’m about to come to you in a thick cloud in order that the people will hear me talking with you so that they will always trust you.” Moses told the LORD what the people said

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.