Va'era 

וארא

In contrast to the previous plagues, Pharaoh’s magicians were unable to replicate the plague of lice. The Sages disagree concerning the reason for this.  

“The magicians did so with their artifices to draw out the lice, but they could not” (Exodus 8:14). Rabbi Elazar said: From here one can derive that a demon [through whose power the Egyptians’ magic worked] is unable to create an item less than the bulk of a barley kernel. The Rabbis say: It cannot create any item, even one the size of a camel; rather, the magicians can gather together these larger items with their magic but they cannot gather together these smaller items. “The magicians said to Pharaoh: It is the finger of God” (8:15). Once the magicians saw they were unable to generate the lice, they immediately acknowledged that these creatures were the creation of God and not the creation of demons, and they would no longer attempt to liken themselves to Moses and continue replicating the plagues. <source>(<i>Shemot</i> <i>Rabba</i> [Shinan] 10)  </source>