Musannaf Ibn Abi Shayba 7:426

Abu Khalid narrated to us, from Ash'ath, from al-Mughirah ibn Abdullah al-Yashkuri, from Abdullah ibn al-Harith, who said: Whoever breaks his fast for one day of Ramadan intentionally, without being on a journey or being ill, shall never be able to make it up, even if he were to fast for his entire life.