I had assumed that Allen Dulles had thrown out the line...

there is no such thing as coincidence.

Dulles, at one point, was in charge of American counterintelligence.

While trying to source the quote (I still haven't cataloged The Craft of Intelligence in my reference DB, so it's floating around my computer somewhere), I ran across this Wiki article, [https://en.wikipedia.org/wiki/The_Moscow_rules], and I thought I'd share it, and a few other "maxims" of intelligence work with you.

The Moscow Rules

• Assume nothing.
• Never go against your gut.
• Everyone is potentially under opposition control.
• Do not look back; you are never completely alone.
• Go with the flow, blend in.
• Vary your pattern and stay within your cover.
• Lull them into a sense of complacency.
• Do not harass the opposition.
• Pick the time and place for action.
• Keep your options open.

Other selected "maxims" from the Wiki article:

• Murphy is right. (i.e., "What can go wrong, will go wrong, and at the worst possible moment.")
• Any operation can be aborted. If it feels wrong, it is wrong.
• Maintain a natural pace.
• Build in opportunity, but use it sparingly.
• There is no limit to a human being's ability to rationalize the truth.
• Technology will always let you down.
• Once is an accident. Twice is a coincidence. Three times is an enemy action. (Taken from Ian Fleming's novel Goldfinger)
• Do not attract attention, even by being overly careful.

Allen Dulles Quotes

It seems I was wrong in my attribution of the opening quote to Dulles. Here are some applicable quotes from his end, though. (Source)

• The measure of success is not whether you have a tough problem to deal with, but whether it is the same problem you had last year.
• Once - many, many years ago - I thought I made a wrong decision. Of course, it turned out that I had been right all along. But I was wrong to have thought that I was wrong.
• The ability to get to the verge without getting into the war is the necessary art. If you try to run away from it, if you are scared to go to the brink, you are lost.
• Of all tasks of government the most basic is to protect its citizens against violence.

Other "Proverbs"

• It is essential to seek out enemy agents who have come to conduct espionage against you and to bribe them to serve you. Give them instructions and care for them. Thus doubled agents are recruited and used.
  Sun Tzu
  Read more at: https://www.brainyquote.com/quotes/sun_tzu_138182?src=t_espionage

• Espionage and counterespionage go together like horse and carriage.
  Charles McCarry
  Read more at: https://www.brainyquote.com/quotes/charles_mccarry_739198?src=t_espionage

• Spying is waiting.
  John le Carré, The Russia House

OK, I had to end with a pic from one of the dumbest movies in history, Spies Like Us