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.'" (IT)
English reference (IT): Vol. 1, Book 1, Hadith 264 (IT)
Arabic reference (IT): Book 1, Hadith 276 (IT)