Bilbo began to whistle loudly and to forget about the night before. In fact he was just sitting down to a nice little second breakfast in the dining-room by the open window, when in walked Gandalf.
“My dear fellow,” said he, “whenever are you going to come? What about an early start?—and here you are having breakfast, or whatever you call it, at half past ten! They left you the message, because they could not wait.”
“What message?” said poor Mr. Baggins all in a fluster.
“Great Elephants!” said Gandalf, “you are not at all yourself this morning—you have never dusted the mantelpiece!”
“What’s that got to do with it? I have had enough to do with washing up for fourteen!”
“If you had dusted the mantelpiece, you would have found this just under the clock,” said Gandalf, handing Bilbo a note (written, of course, on his own note-paper).
J. R. R. Tolkien: The Hobbit, Chapter II, Roast Mutton
Bilbo's mantelpiece clock with Thorin's letter just under it.
I really wanted to do this one (even if belatedly), because I had very clear idea what to draw for it.
Perhaps because this is, as far as I know, the only place in Tolkien's works where a clock is mentioned.