Earliest appearance in print, attributed to Thomas Jefferson: See above.
Comments: This statement has not been found in the writings of Thomas Jefferson. Most sources connect it to the Louisiana Purchase. Many claim that it was a comment made by Jefferson to Congress in announcing the purchase (likely referring to Jefferson's Third Annual Message to Congress), but no such comment appears in that document.2
1. Note: The quotation usually appears without "I;" e.g. "Thomas Jefferson said that he 'stretched the Constitution until it cracked.'"