What's the source for Jefferson's quote: "If a law is unjust, a man is not only right to disobey it, he..."?

Many people repeat the quote "If a law is unjust, a man is not only right to disobey it, he is obligated to do so", ostensibly by Thomas Jefferson, but I have not found a source for it...I need to know where/when he said it, so I can cite it in a daily quotation.
Update: I need WHERE AND WHEN Jefferson said/wrote it, not simply a link to someone repeating the quote with no citation at all.

So a link to liberty quotes, who attribute it only to "Thomas Jefferson", not the name and date of the book, speech, or whatever, does me no good.
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