Source, in full
In an address of 13 October 1896, published in Proceedings of the Liverpool Geological Society, Volumes 8-9 (1900), p. 28 Thomas Mellard Reade attributed this statement to John Playfair, citing it to his Illustrations of the Huttonian Theory of the Earth (1802); it has not yet been located in the volumes of that work presently available online, and was perhaps a summation or paraphrase, as Reade's statement reads
Notes from Wikiquote
It was well observed by Playfair, some 90 years ago, that "a theory that explains everything explains nothing."