This phrase is often attributed to Francis Bacon, in his Meditationes Sacrae (1597).^[1] Thomas Jefferson used the phrase at least twice:

"this last establishment will probably be within a mile of Charlottesville, and four from Monticello, if the system should be adopted at all by our legislature who meet within a week from this time, my hopes however are kept in check by the ordinary character of our state legislatures, the members of which do not generally possess information enough to percieve the important truths, that knolege is power, that knolege is safety, and that knolege is happiness." - Thomas Jefferson to George Ticknor, 25 November 1817^[2]

"All the states but our own are sensible that knolege is power."
- Thomas Jefferson to Joseph Cabell, 22 January 1820^[3]

Footnotes

1. ↑ John Bartlett, Familiar Quotations, 10th ed. 1919. The entry for this quotation is available online at http://www.bartleby.com/100/139.39.html.
2. ↑ Polygraph copy at Historical Society.
3. ↑ Ford, 12:155. Polygraph copy at the Library of Congress.