Where did “Cleanliness is next to Godliness” originate?

I've often heard the phrase "Cleanliness is next to godliness" used but as far as I know, while there's nothing intrinsically wrong with the notion, in spite of mentioning God the phrase doesn't have a Biblical basis. Where did the phrase come from and does it have its roots in Christianity or somewhere else?

John Wesley in one of his sermons indicated that the proverb was already well known in the form we use today. Wrote Wesley: 'Slovenliness is no part of religion.'Cleanliness is indeed next to Godliness.

(from PhraseFinder, which has a useful entry too long to quote)