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. This is the view maintained by Al-Shaafi`ee and Imaam Ahmad based on the Hadeeth related by Al-Bukhaaree and Muslim to the effect that the Prophet (peace be upon him) said: Anyone who forgets that they are fasting and eats or drinks should complete their Sawm, for it is Allaah Who fed them and gave them something to drink. According to another wording: If a fasting person eats or drinks something forgetfully, it is something with which Allaah provided him and he does not have to make up for the fast of that day. (Related by Al-Daaraqootnee who ranked its Sanad [chain of narrators] to be authentic)… read more here.