Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "He who is asked something he knows and conceals it will have a bridle of fire put on him on the Day of Resurrection."
This Hadīth carries a stern warning about the concealment of knowledge and that if a person is asked about something that concerns the questioner as regards his religious matters, and he is duty-bound to answer him, yet he fails to answer and clarify the matter to him, Allah, the Almighty, will punish him in the Hereafter by putting a bridle of fire over his mouth. However, if the questioner poses the question for some other reason, like testing the one he asks, the asked one has the choice to answer him or not, without falling under the warning set out in the Hadīth.