Musar zu Schemot 5:1

This was also the subject Moses and Pharaoh debated, i.e. when Pharaoh challenged "who is G–d that I should listen to Him!" (Exodus 5,2). Moses, on the other hand, said "thus said the Lord G–d of Israel, let My people go!" (Exodus 5,1). He argued the existence of G–d and the fact that the people of Israel have a close relationship with Him. This is why he described Him as "the G–d of Israel." From a philosophical point of view, Pharaoh challenged Moses, saying that even if a חידוש העולם were to occur- something he denied, not having heard about it, surely it could not be claimed that a special relationship already existed between this G–d and Israel, seeing it is claimed that the world was entering a new beginning! Since G–d is self-sufficient, any relations He establishes with anyone are for that party's benefit. Since this is so, how could Israel be special? He must dispense favours equally! Surely, He would also have commanded me to serve Him! His failure to do so makes Moses' claim unbelievable. Therefore, Pharaoh would not allow Israel to go and serve G–d in the desert.