Quotable Jefferson, ed. Kaminski (searching under "guns")
Earliest known appearance in print: No appearances in print found. 
Earliest known appearance in print, attributed to Thomas Jefferson: See above.
Other attributions: None known.
Status: We have not found any evidence that Thomas Jefferson said or wrote, "Those who hammer their guns into plows will plow for those who do not."
↑ To establish the earliest appearance of this phrase in print, the following sources were searched for the phrase, "Those who hammer their guns into plows will plow for those who do not.": Google Books, Google Scholar, Amazon.com, Internet Archive, JSTOR.