Exodus 3:10-12

But come, and I will send thee to Pharao, that thou mayst bring forth my people, the children of Israel, out of Egypt.

And Moses said to God: Who am I that I should go to Pharao, and should bring forth the children of Israel out of Egypt?

And he said to him: I will be with thee; and this thou shalt have for a sign that I have sent thee: When thou shalt have brought my people out of Egypt, thou shalt offer sacrifice to God upon this mountain.