Many orthodox people speak as though it were the business of sceptics to disprove received dogmas rather than of dogmatists to prove them. This is, of course, a mistake. If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense.

Bertrand Russell (1872-1970) English mathematician and philosopher

“Is There a God?” (1952)
    (Source)

Essay commissioned by Illustrated magazine in 1952, but never published there. First publication in Russell, Last Philosophical Testament, 1943-68 (1997) [ed. Slater/Köllner].

I believe this is the earliest (chronologically) reference by Russell to his teapot analogy.