Chasidut zu Schemot 7:9

Exodus 7,9. “if Pharaoh will say to you: ‎‎‘identify yourselves by means of a miracle, say to ‎Aaron to take his staff etc.;’” at first glance ‎the word ‎לכם‎ used by Pharaoh here was superfluous as ‎Moses and Aaron did not need to identify themselves ‎to each other. He should simply have said: ‘prove your ‎mission by performing a miracle!’”
The Ari z’al writes that it is a rule that ‎every human being is convinced that his words will ‎make a favourable impression on his Creator, and as a ‎result further G’d’s dispensing of benefits to His ‎people. Because of this belief in the power of speech, ‎every person has to be extremely careful not to abuse ‎this power of speech by talking nonsense, or worse. ‎Pharaoh invited, or challenged Moses and Aaron to say ‎things which would serve as proof that due to their ‎words their G’d of whose existence Pharaoh was well ‎aware, would perform acts that would impress him. ‎When Moses therefore instructed Aaron to take his ‎staff, throw it to the ground and it would turn into a ‎snake, he complied with exactly what Pharaoh had ‎demanded of them. The miracle was the result of ‎Moses’ speaking to Aaron.‎‎