Sahih al-Bukhari 1933
Sahih al-Bukhari · Book 30: Fasting · If a person observing Saum ate or drank forgetfully
Narrated Abu Huraira:
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 " .
The Prophet (ﷺ) 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."