Hilchos Rechilus, chapter 9, section 14

Suppose something wrong was done to Reuven, but he does not know who did it. Reuven comes and asks Shimon: "Who did this?"

Even though Shimon understands that Reuven suspects him, he may not say who did it, even if he saw it himself. Instead he should answer: "I did not do this."

(Unless it is something he would have had to tell Reuven on his own even if Reuven had not asked him — for example, when all the details listed at the start of this chapter are met.)