'Will you have some whiskey and water?' Holmes asked Gregson in A Study in Scarlet, as Gregson sat down to explain why he had arrested Arthur Charpentier. Gregson never finished it as knocked over the remainder of the drink following Lestrade's surprise announcement of the murder of Joseph Stangerson.
Whiskey-pegs
Abel White would sit on his veranda drinking whisky-pegs and smoking cheroots, while the country was in a blaze about him. A 'peg' is a measure, originally placed inside a tankard to make sure that when the drink is passed on the next drinker doesn't drink too much. (The Sign of Four)
Holmes gave Jonathan Small a glass of Whisky and water while Small was telling the story of the Agra Treasure. (The Sign of Four)