Musnad Ahmad 30:3290

Ibn Numayr narrated to us, he said: 'Umarah ibn Zadhan narrated to us, from 'Ali ibn al-Hakam, from 'Ata' ibn Abi Rabah, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever is asked about knowledge that he knows, and then conceals it, will be bridled on the Day of Resurrection with a bridle of fire."