Sahih al-Bukhari, Volume 3, Book 31, Hadith 154 (USC-MSA) ⓘ
Narrated Abu Huraira: The Prophet said, "If somebody eats or drinks forgetfully then he should complete his fast, for what he has eaten or drunk, has been given to him by Allah."
حَدَّثَنَا عَبْدَانُ، أَخْبَرَنَا يَزِيدُ بْنُ زُرَيْعٍ، حَدَّثَنَا هِشَامٌ، حَدَّثَنَا ابْنُ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا نَسِيَ فَأَكَلَ وَشَرِبَ فَلْيُتِمَّ صَوْمَهُ، فَإِنَّمَا أَطْعَمَهُ اللَّهُ وَسَقَاهُ " .
also referenced under: Book 30, Hadith 40 (In-Book) & Hadith 1933 (DarusSalam)