“It's my rule never to lose me temper till it would be dethrimental to keep it.”

Source: King Lear

Source: A Little Princess

Source: Toll the Hounds

p 9, reflecting on his father's near-drowning off the Australian coast
Achieving The Impossible (2010)

This maxim (perhaps of gambling or horse racing origin) is widely attributed to Warren Buffett and, as such, has traditionally been cited in print; notably, it was attributed (perhaps facetiously) to him by Mary Buffett in, The Tao of Warren Buffett. A more uncommon, less well known version, and perhaps one with a more lasting credibility (or certainly with a higher degree of checkability), would be: "The first rule is don't lose, and the second rule is never forget the first rule." This version was noted by Steve Forbes in a friendly meeting in Omaha, in an article published as: Jay-Z, Buffett and Forbes on Success and Giving Back. This article is available on the Forbes website, published on September 23, 2010.
Disputed
Variant: Rule No. 1: Never lose money. Rule No. 2: Never forget rule No. 1.

Source: Frost Burned

Nahj al-Balagha, Letter 53: An order to Malik Al-Ashtar

Source: Frostbite