Not the final source, but possibly this is the quote:
“So you’re jealous of me, very well; come out into the courtyard. I'll have twenty shots at you at thirty paces. If I don't hit you, the whole house and everything in it is yours.” –A.G. MacDonnell, Napoleon and his Marshalls (1934)
Quoted in: Max Hastings, The Oxford Book of Military Anecdotes. P. 200