Citing sources of problems in assignments This question was raised by Dave Clarke here.

When a textbook author approaches a topic in a novel way or presents a particularly interesting example, I believe that a teacher who creates lecture notes using this novel approach or interesting example would be doing the right thing to cite the originator of the approach or the example.  Similarly, whenever I copy a clever (and clearly unique) problem from a textbook and give it as a question in an exam or an assignment, I try my best to indicate (in the exam or assignment paper itself) the source of the original problem.

Does anyone know of any written document indicating whether or not it is considered unethical to copy a published problem and put it in an exam or an assignment without citing it?
I'm a textbook author. I agree that it's hard to formulate a cut-and-dried answer to your question. One criterion to consider: If a colleague would compliment you on a copied problem or question because it was particularly clever or insightful, you should probably consider a citation. Stated differently, if you're getting academic credit (even if informal) for the contribution, cite the source.