Source, in full
Fable XXI, "The Rat-catcher and Cats". Compare: "Potter is jealous of potter, and craftsman of craftsman; and poor man has a grudge against poor man, and poet against poet", Hesiod, Works and Days, 24; "Le potier au potier porte envie" (translated: "The potter envies the potter"), Bohn, Handbook of Proverbs; also in Arthur Murphy, The Apprentice, act iii