Exodus 3:12, 18

He said, “But I will be with you, and this shall be the sign for you, that I have sent you: when you have brought the people out of Egypt, you shall serve God on this mountain.”

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