If anyone forgets that he is fasting and eats or drinks he should complete his fast, for it is only Allah Who has fed him and given him drink.  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ نَسِيَ وَهُوَ صَائِمٌ فَأَكَلَ أَوْ شَرِبَ فَلْيُتِمَّ صَوْمَهُ فَإِنَّمَا أَطْعَمَهُ اللَّهُ وَسَقَاهُ