Exodus 3:12, 18

So He said, “I will certainly be with you. And this shall be a sign to you that I have sent you: When you have brought the people out of Egypt, you shall serve God on this mountain.”

Then they will heed your voice; and you shall come, you and the elders of Israel, to the king of Egypt; and you shall say to him, ‘The LORD God of the Hebrews has met with us; and now, please, let us go three days’ journey into the wilderness, that we may sacrifice to the LORD our God.’