If you can't stand the heat, get out of the kitchen
Don't persist with a task if the pressure of it is too much for you. The implication being that, if you can't cope, you should leave the work to someone who can.
This is widely reported as being coined by US President Harry S. Truman. That's almost correct, but in fact Truman was known to have used it at least as early as 1942 - before becoming president. Here's a citation from an Idaho newspaper The Soda Springs Sun, from July that year:
"Favorite rejoinder of Senator Harry S. Truman, when a member of his war contracts investigating committee objects to his strenuous pace: 'If you don't like the heat, get out of the kitchen'."
He used a version slightly nearer the one most often used nowadays, in 1949, after becoming president, when warning his staff not to concern themselves over criticism about their appointments:
"I'll stand by [you] but if you can't take the heat, get out of the kitchen."
Truman was well-known as a plain-speaker, in a way that politicians in our more media-sensitive age rarely are. This was celebrated by Merle Miller, who published a set of interviews with him - called Plain Speaking: An Oral Biography of Harry S. Truman, in 1974. It includes this unambiguous gem, which would hardly get past the presidential spin-machine these days:
"I didn't fire him [General MacArthur] because he was a dumb son of a bitch, although he was, but that's not against the law for generals. If it was, half to three quarters of them would be in jail."
The other phrase associated with Truman that has entered our language is 'the buck stops here'.
Another linguistic quirk concerning Truman is the spelling of his name. The 'S' isn't short for anything - it is just 'S', and by normal grammatical convention it wouldn't be followed by a full stop (period). Truman always signed his name with a full stop though.
See also: the List of Proverbs.
See other phrases that were coined in the USA.