It's not possible to know exactly when or if it happened, but some estimates place it at around 1400 BCE
This was after God told Moses to throw his staff and it becamea snake. It was god promising Moses that he is the same God.
According to the Bible around 1441 BC. The Bible confidently states that Moses led the Israelites out of Egypt 480 years before the fourth year of Solomon's reign, which would therefore be...
Possibly it is that God spoke to Moses.
Intro to the Torah Book of Shmot or Exodus
Reading Parashat Shmote Exodus Chapter 1
Good Things Happen to Good People
Author Isabel Gillies Discusses Her Memoir Happens Every Day