Revenons à nos moutons. Return we to our subject. The phrase is taken from an old French play, called L' Avocat, by Patelin, in which a woolendraper 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 defendant's 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).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894