Abu Huraira related that Rasulullah said: If anyone omits his fast even for one day in Ramadan without a concession or without being ill, then if he were to fast for the rest of his life he could not make up for it (Bukhari).
Abu Huraira related that the Prophet said: If a person does not avoid false talk and false conduct during Siyam, then Allah does not care if he abstains from food and drink (Bukhari, Muslim).