I sincerely believe that banking institutions are more dangerous to our liberties than standing armies. Already they have raised up a money aristocracy that has set the government at defiance. The issuing power should be taken from the banks and restored to the people to whom it properly belongs.

The earliest known appearance of this statement is from 1895 (Joshua Douglass, "Bimetallism and Currency", American Magazine of Civics, 7:256). It is apparently a combination of paraphrases or approximate quotations from three separate letters of Jefferson (longer excerpts in sourced section)

I sincerely believe, with you, that banking institutions are more dangerous than standing armies... | Letter to John Taylor (1816) | The bank mania...is raising up a moneyed aristocracy in our country which has already set the government at defiance... | Letter to Josephus B. Stuart (1817) | Bank paper must be suppressed, and the circulating medium must be restored to the nation to whom it belongs. | Letter to John W. Eppes (1813)