It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Whoever conceals knowledge that he knows will be bridled with a bridle of fire on the Day of Resurrection." He said: "This concerns testimony: a man is called to give testimony or is not called to it, and he knows it but does not guide the one entitled to it towards it; this is that knowledge."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10799