Exodus 19:8

5 If now you will truly give ear to my voice and keep my agreement, you will be my special property out of all the peoples: for all the earth is mine:

6 And you will be a kingdom of priests to me, and a holy nation. These are the words which you are to say to the children of Israel.

7 And Moses came and sent for the chiefs of the people and put before them all these words which the Lord had given him orders to say.

8 And all the people, answering together, said, Whatever the Lord has said we will do. And Moses took back to the Lord the words of the people.

9 And the Lord said to Moses, See, I will come to you in a thick cloud, so that what I say to you may come to the ears of the people and they may have belief in you for ever. And Moses gave the Lord word of what the people had said.

10 And the Lord said to Moses, Go to the people and make them holy today and tomorrow, and let their clothing be washed.

11 And by the third day let them be ready: for on the third day the Lord will come down on Mount Sinai, before the eyes of all the people.