Source, in full
sometimes to Lord Rayleigh or Simon Newcomb. It is a fact that Kelvin did not believe in heavier-than air flight , but there is no reliable source that he or another physicist from 19th century said it was impossible from a scientific point of view.
Notes from Wikiquote
The oldest known source is the book from Chris Morgan Facts and fallacies: a book of definitive mistakes and misguided predictions (1981)