Exodus 3:12, 18

And God said, “I will be with you. And this will be the sign to you that it is I who have sent you: When you have brought the people out of Egypt, you will worship God on this mountain.”

“The elders of Israel will listen to you. Then you and the elders are to go to the king of Egypt and say to him, ‘The LORD, the God of the Hebrews, has met with us. Let us take a three-day journey into the wilderness to offer sacrifices to the LORD our God.’