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. (Using translation from Muslim 1155)  

عَنِ النَّبِيِّ ﷺ قَالَ «مَنْ نَسِيَ وَهُوَ صَائِمٌ فَأَكَلَ أَوْ شَرِبَ فَلْيُتِمَّ صَوْمَهُ فَإِنَّمَا أَطْعَمَهُ اللَّهُ وَسَقَاهُ»  

[Machine] The Messenger of Allah ﷺ said, "If any of you forgets and eats or drinks while fasting, let him complete his fast for it is Allah who has provided him with food and drink." Abu Muhammad said, "The people of Hijaz say he should make up for that day, but I say he should not."  

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا أَكَلَ أَحَدُكُمْ أَوْ شَرِبَ نَاسِيًا وَهُوَ صَائِمٌ ثُمَّ ذَكَرَ فَلْيُتِمَّ صِيَامَهُ فَإِنَّمَا أَطْعَمَهُ اللَّهُ وَسَقَاهُ» قَالَ أَبُو مُحَمَّد أَهْلُ الْحِجَازِ يَقُولُونَ يَقْضِي وَأَنَا أَقُولُ لَا يَقْضِي