watched pot never boils, a

What's the use of watching? A watched pot never boils.
[1848 Gaskell Mary Barton II. xiv.]

Don't you know that vulgar old proverb that says that ‘a watched pot never boils’?
[1880 M. E. Braddon Cloven Foot III. viii.]

‘He [Mussolini] is waiting to see how the next battle turns out,’ they said. ‥‘A watched pot never boils,’ they said—only this one finally did.
[1940 C. Boothe Europe in Spring x.]

‘Whoever said, “A watched pot never boils” obviously didn't own a microwave.’
[2002 Washington Post 26 Apr. C10 (Born Loser comic strip)]

Related to: patience and impatience

Bibliography of major proverb collections and works cited from modern editions is available here.

Anxious waiting does not speed up matters, as in Stop running downstairs for every mail delivery--a watched pot never boils, you know. This hyperbolic adage reflects the experience of anyone who has ever been in a hurry to bring water to a boil, which eventually occurs but can seem to take forever. [Mid-1800s]