It was narrated that Abu Hurairah said:
"The Messenger of Allah said: 'Whoever is asked about knowledge that he has and he conceals it, will be bridled on the Day of Resurrection with reins of fire.'" (ID)
English reference (ID): Vol. 1, Book 1, Hadith 266 (ID)
Arabic reference (ID): Book 1, Hadith 278 (ID)