Source, in full
Comment made at a crisis point during the Battle of Waterloo at about 5.45 pm on 18 June. The Military Maxims of Napoleon by Napoleon Bonaparte, David G. Chandler, William E. Cairnes , p. 143 Alternatively wording may have been "Night or the Prussians must come": quoted by David Howarth, page 162, "Waterloo: Day of Battle", ISBN=0-88365-273-0