Whoever forgets that he is fasting and eats or drinks, let him complete his fast, for it was Allah who fed him and gave him to drink. [Al-Bukhari]
"«مَنْ نَسِيَ وَهُوَ صَائِمٌ فأَكل أو شَرِب، فَلْيُتِمَّ صَوْمَهُ، فَإِنَّمَا أَطْعَمَهُ الله وَسَقَاهُ». [صحيح.] - [متفق عليه."