“When they raid the whorehouse, they take all the girls” is a Wall Street saying, meaning that when stocks start to strongly fall or to strongly rise, the market takes all the stocks in the trend. The saying has been cited in print since at least 1973: “...in Wall Street’s timeworn analogy, ‘When the police raid a bawdy house, they take all the girls.’”
     
A version of the Wall Street adage with a piano player—“When you raid a whorehouse, take the piano player, too, because no one is entirely innocent”—has been cited in print since at least 1980.

     
Google Books
The Mutual Fund Trap
By John Lawrence Springer
Chicago, IL: Regnery
1973  
Pg. 165:
The record shows quite plainly that you cannot count on a fund to choose stocks that will go up while the market turns down; or, in Wall Street’s timeworn analogy, “When the police raid a bawdy house, they take all the girls.”
     
Google Books
EgoSpeak: Why no one listens to you
By Edmond G. Addeo and Robert E. Burger
London: Bantam
1974
Pg. 15:
“Sounds like they squeezed the shorts and then fell out of bed.”
“Not really. When they raid the house, they take all the girls!”
“What a bucket shop!”
Two reprobates in a Nevada house of ill repute? No. Just two stockbrokers playing a variation of JobSpeak for the benefit of a third party at San Francisco’s Iron Pot Restaurant.
   
Google Books
The Dow Jones-irwin Guide to Modern Portfolio Theory .
By Robert L. Hagin
Homewood, IL: Dow Jones-Irwin
1979
Pg. 168:
There is considerable evidence that securities tend to move with the market. Whether based on such time-honored Wall Street aphorisms as “when they raid the brothel, they take all the girls,” or Brealey’s less colorful statement, “when the wind of recession blows, there are few companies that do not lean with it,” it is true thatin major market moves, most securities move in the same direction.
   
Google Books
How Charts Can Help You in the Stock Market
By William L. Jiler
Burlington, VT: Fraser Pub. Co.
1990
Pg. 157:
As the old Wall Street saying has it, “When they raid the house, they take all the girls — and the piano player.” In other words, when the market is in a strong downtrend or uptrend, it will carry with it, sooner or later, a large majority of stocks, including many that, on their own merits, would be behaving quite differently.
 
Google Books
The Right Time The Right Place
By Charles Wohlstetter
New York, NY: Applause
1997
Pg. 22:
In the months that followed, everything, good and bad, was in free fall, confirming the age-old adage that when they raid the whorehouse they take all the girls. There were an unprecedented sixteen million shares traded on that day as the Dow Jones Average plummeted more than thirty points.
 
Google Books 
Done Deals:
Venture capitalists tell their stories
By Udayan Gupta
Boston, MA: Harvard Business School Press
2000
Pg. 188:
My old boss at Fireman’s Fund used to say, “When they raid the house, they take all the girls. They don’t distinguish between good girls and bad girls, they take them all.” In this industry that means that all the stocks go down—good stocks go down with the bad stocks.
   
Google Books
Understanding Wall Street
Fifth Edition
By Jeffrey B. Little and Lucien Rhodes
New York, NY: McGraw-Hill
2010
Pg. 279:
BEAR MARKET: “When they raid the house, they take all the girls . . . but the madam and the piano player get bailed out first.”