Sunan Ibn Majah, Book 7, Hadith 1743 (In-Book) ⓘ
It was narrated from Abu Hurairah that the Messenger of Allah said: "Whoever eats out of forgetfulness while fasting, let him complete his fast, for it is Allah Who has fed him and given him to drink."
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ عَوْفٍ، عَنْ خِلاَسٍ، وَمُحَمَّدِ بْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " مَنْ أَكَلَ نَاسِيًا وَهُوَ صَائِمٌ فَلْيُتِمَّ صَوْمَهُ فَإِنَّمَا أَطْعَمَهُ اللَّهُ وَسَقَاهُ " .
also referenced under: Hadith 1673 (Reference) & Volume 2, Book 7, Hadith 1673 (DarusSalam)