According to Exodus 7, Moses’s staff became a snake when Aaron threw it down before Pharaoh.
The sign occurred during Moses and Aaron’s confrontation with Pharaoh, before the ten plagues unfolded. Aaron cast down the staff at Moses’s command, and it became a serpent. Pharaoh then summoned Egyptian wise men and magicians, whose staffs also appeared to become serpents, but Aaron’s staff swallowed theirs.
The episode establishes the staff as a sign of divine authority and introduces the contest between Moses’s mission and Pharaoh’s court. In the biblical account, the sign is performed by Aaron even though Moses is the central leader.
This event should be separated from the later use of the staff at the sea and during the wilderness journey. The transformation before Pharaoh is specifically the change into a snake.