Мидраш к Шмот 7:22

And when the people saw that Moses delayed to come down (Exod. 32:1). The word boshesh (“delayed”) indicates that it was the sixth hour of the day. Forty thousand people had assembled to leave Egypt with the Israelites, and among them were two Egyptians named Jannes and Jambres, who had performed magical feats for Pharaoh, as it is written: And the magicians of Egypt in like manner with their arts (Exod. 7:22). All of them gathered about Aaron, as is said: And the people gathered themselves together unto Aaron and said (ibid. 32:11).