It was narrated that Abu Sa'eed Al-Khudri said:
"The Messenger of Allah said: 'Whoever conceals knowledge which Allah has made beneficial for mankind's affairs of religion, Allah will bridle him with reins of fire on the Day of Resurrection." (IT)
English reference (IT): Vol. 1, Book 1, Hadith 265 (IT)
Arabic reference (IT): Book 1, Hadith 277 (IT)