E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Revenons à nos moutons. Return we to our subject. The phrase is taken from an old French play, called LAvocat, by Patelin, in which a woollen-draper charges a shepherd with stealing sheep. In telling his grievance he kept for ever running away from his subject; and to throw discredit on the defendants attorney, accused him of stealing a piece of cloth. The judge had to pull him up every moment with, Mais, mon ami, revenons à nos moutons (What about the sheep, tell me about the sheep, now return to the story of the sheep).