E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Lose the Horse or win the Saddle.
Everything or nothing. Aut Cæsar, aut nullus. A man made the bet of a horse that another could not say the Lords Prayer without a wandering thought. The bet was accepted, but before half-way through the person who accepted the bet looked up and said, By-the-bye, do you mean the saddle also?