Musnad Ahmad 32:1230

Hujayn ibn al-Muthanna narrated to us, Isra'il narrated to us, from Abu Ishaq, from Burayd ibn Abi Maryam, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever asks Allah for Paradise three times, Paradise says: 'O Allah, admit him into Paradise.' And whoever seeks refuge with Allah from the Fire three times, the Fire says: 'O Allah, protect him from the Fire.'"