Whenever you feel a warmth of temper rising, check it at once, and suppress it, recollecting it will make you unhappy within yourself, and disliked by others. Nothing gives one person so great advantage over another, as to remain always cool and unruffled under all circumstances.

Thomas Jefferson (1743-1826) American political philosopher, polymath, statesman, US President (1801-09)

Letter (1816-05-21) to Francis Eppes
    (Source)

Often updated as "Nothing gives one person so much advantage over another as to remain always cool and unruffled under all circumstances."