From Thomas Jefferson Encyclopedia
The following story about a meeting between Thomas Jefferson and George Washington has been repeated many times:
It is said that when Jefferson returned from France he was breakfasting with Washington, and asked him why he agreed to a Senate.
"Why," said Washington, "did you just now pour that coffee into your saucer before drinking it?"
"To cool it," said Jefferson; "my throat is not made of brass."
"Even so," said Washington, "we pour our legislation into the Senatorial saucer to cool it."
To date, no evidence has surfaced that such a conversation actually took place. The earliest known appearance of this story is in Harper's New Monthly Magazine in 1884 (the transcription above is from this source). It was repeated by M.D. Conway in his Omitted Chapters of History Disclosed in the Life and Papers of Edmund Randolph, first published in 1888. Since then, the story has appeared many times in print, usually prefaced by the phrase, "the story goes..." or similar.