Sunan Ibn Majah 262
The Book of the Sunnah
One who is asked about knowledge but conceals it
It was narrated that 'Abdur-Rahman bin Hurmuz Al-A'raj heard Abu Hurairah say: "By Allah, were it not for two Verses in the Book of Allah, I would never have narrated anything from him, meaning from the Prophet, were it not for the Words of Allah: Verily, those who conceal what Allah has sent down of the Book, and purchase a small gain therewith (of worldly things), they eat into their bellies nothing but fire. Allah will not speak to them on the Day of Resurrection, nor purify them, and theirs will be a painful torment. Those are they who have purchased error at the price of guidance, and torment at the price of forgiveness. So how bold they are (for evil deeds which will push them) to the Fire.'"