Question: If one eats or drinks forgetfully during a day in Ramadaan, should one make up for the Sawm (Fast) of that day? The Messenger (peace be upon him) said: “Anyone who forgets that they are fasting and eats or drinks…”? He (peace be upon him) also said: My Ummah (nation based on one creed) has been pardoned for what they do by mistake, forgetfully, or under duress.
Answer: Anyone who eats or drinks forgetfully during the daytime in Ramadaan is not sinful and should complete their Sawm and not break it. Moreover, they need not make up for the Sawm of the day in question, according to the more correct of the two opinions maintained by scholars... read more here.