“The essence of strategy is choosing what not to do.”

For My Legionaries: The Iron Guard (1936), Politics

Source: The Theory of Gambling and Statistical Logic (Revised Edition) 1977, Chapter Three, Fundamental Principles Of A Theory Of Gambling, p. 43

Source: 2010s, Free Will (2012), p. 39

"What is strategy?," 1996
Context: There's a fundamental distinction between strategy and operational effectiveness. Strategy is about making choices, trade-offs; it's about deliberately choosing to be different. Operational effectiveness is about things that you really shouldn't have to make choices on; it's about what's good for everybody and about what every business should be doing.

Often given as a saying of Aristotle with no reference.
Disputed

Source: Good Strategy Bad Strategy, 2011, p. 20
Context: Having conflicting goals, dedicating resources to unconnected targets, and accommodating incompatible interests are the luxuries of the rich and powerful, but they make for bad strategy. Despite this, most organizations will not create focused strategies. Instead, they will generate laundry lists of desirable outcomes and, at the same time, ignore the need for genuine competence in coordinating and focusing their resources. Good strategy requires leaders who are willing and able to say no to a wide variety of actions and interests. Strategy is at least as much about what an organization does not do as it is about what it does.

Entrepreneur "Strategy is Not a To-do List" https://www.entrepreneur.com/article/238513 October 16, 2014.