Va'era 

וארא

In order for the first three plagues to commence, it was necessary to use a staff to strike Egypt’s water and its dust. The Sages relate that because the water and dust of Egypt provided benefit to Moses in the past, it was not he who generated these plagues but rather his brother, Aaron.  

Why was the water not struck by Moses to generate the plagues of blood and frogs? The Holy One, blessed be He, said to Moses: Concerning the water that protected you when you were cast into the Nile, it is inappropriate for it to be hit by you. I take an oath by your life that it will be hit only by Aaron.  “The Lord said to Moses: Say to Aaron: Extend your staff and strike the dust of the earth and it will become lice” (Exodus 8:12). Rabbi Tanhum said: The Holy One, blessed be He, said to Moses: Concerning the soil that defended you when you killed the Egyptian and buried him in the sand, it is inappropriate for it to be hit by you. Therefore, these three plagues of blood, frogs, and lice were effected by Aaron. <source>(<i>Shemot</i> <i>Rabba</i> 10) </source>