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.'" (ES)
English reference (ES): Vol. 1, Book 1, Hadith 266 (ES)
Arabic reference (ES): Book 1, Hadith 278 (ES)