Musnad Ahmad 30:930

Abu Kamil narrated to us, Hammad narrated to us, from Ali ibn al-Hakam, from Ata ibn Abi Rabah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever is asked about knowledge and conceals it, he will be bridled with a bridle of fire on the Day of Resurrection."