Yusuf bin Ibrahim said:
'I heard Anas bin Malik say: "I heard the Messenger of Allah say: "Whoever is asked about knowledge and conceals it, it will be bridled on the Day of Resurrection with reins of fire.'" (RU)
English reference (RU): Vol. 1, Book 1, Hadith 264 (RU)
Arabic reference (RU): Book 1, Hadith 276 (RU)