Ibn `Abbas reported that the Prophet, peace and blessings be upon him, said: "The bare essence of Islam and the basics of the religion are three [acts], upon which Islam has been established. Whoever leaves one of them becomes an unbeliever and his blood may legally be spilled. [The acts are:] Testifying that there is no God except Allah, the obligatory prayers, and the fast of Ramadan." (Related by Abu Ya'la and Ad-Dailimi. Adh-Dhahabi and classified as Sahih.)
Abu Hurairah reported that the Messenger of Allah, peace and blessings be upon him, said: "Whoever breaks his fast during Ramadan without having one of the excuses that Allah would excuse him for, then even a perpetual fast, if he were to fast it, would not make up for that day." (Related by Abu Dawud, Ibn Majah, and At-Tirmidhi.)
Al-Bukhari records from Abu Hurairah in Marfu` form: "Whoever breaks the fast of Ramadan without having a legitimate excuse or being ill, he cannot make up for that day, even if he were to undertake a perpetual fast." Ibn Mas`ud has also reported this.
Adh-Dhahabi says: "According to the established believers, anyone who leaves the fast of Ramadan without being sick is worse than a fornicator or an alcoholic. In fact, they doubt his Islam and they suspect that he might be a Zandaqah and one of those who destroy [Islam].