This is most obviously attested to in a nice patch of dialogue from "The Adventure of Shoscombe Old Place," one of the last canonical Homes stories written:
"By the way, Watson, you know something of racing?"One presumes the question was a rhetorical or even a joking one; it's not as if the ever-observant Holmes didn't know this already. Many commentators have speculated that Holmes placed a bet on Silver Blaze, but I am certain that Watson did.
"I ought to. I pay for it with about half my wound pension."
Not that racing was the only form of gambling in which the good doctor indulged. The opening scene in "The Adventure of the Dancing Men" has Holmes deducing that he has decided not to buy South African securities, which would imply a willingness to dabble in the stock market.
And wasn't it a gamble for him to propose to Mary Morstan on such short acquaintance?
But the biggest wager of his life was when he decided to throw in his lot with an eccentric character named Sherlock Holmes in renting a flat at 221B Baker Street. We can only wish that all of Dr. Watson's bets had turned out so well.