Found the following in Jonathan Swift's Letter to a Young Clergyman:
reasoning will never make a man correct an ill opinion, which by reasoning he never acquired
So now RichardKennaway's comment says that the earliest appearance known to Wikiquote is from 1851 ... but it links to Wikiquote's page about Jonathan Swift, with that same quotation. Richard, did you half-edit your comment?
Another month, another rationality quotes thread. The rules are: