Accidentally finding the solution to an exercise I was given an assignment for a graduate course a few days ago, and I had to do some background reading in order to understand what one question was asking. In the process of doing this reading, I discovered a paper that contained the exact question I was working on, but phrased as a statement. A proof of this statement was also included, meaning the solution was now laid out in front of me.

I'm not sure how to approach this situation. I can't simply reference this paper, since telling the instructor "the solution may be found here" misses the point of the exercise. On the other hand, I can't write my solution without providing a citation, since that would be academic dishonesty. What should I do in this situation?
Literature research is a legitimate process in science, they can't blame you for that. Clearly, however, you did not "rediscover" the result. One option would be to tell the tutor that you found a solution in the literature and if s/he could give you an alternative assignment instead.