Exodus 3:12, 18

And He said, “Certainly I will be with you, and this shall be the sign to you that it is I who have sent you: when you have brought the people out of Egypt, you shall worship God at this mountain.”

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