Midrash su Esodo 5:5

And Pharaoh said: Behold, the people of the land are now many (ibid., v. 5). R. Simeon the son of Yohai said: What did he do to them? He began to gnash his teeth furiously against them, and he called out to them, saying: You are idlers (nirpim) (ibid., v. 17). Nirpim means “uncleanness.” May his bones be ground up. “So you claim that you are holy men? and you say: Let us go (ibid., v. 17)?” R. Joshua the son of Levi said: The tribe of Levi was exempted from hard labor, and therefore Pharaoh said to them: “Perhaps it is because you are exempted from hard labor that you dare say: Let us go and sacrifice to our God (ibid., v. 8) (Get you unto your burdens.)