"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."  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ أَكَلَ نَاسِيًا وَهُوَ صَائِمٌ فَلْيُتِمَّ صَوْمَهُ فَإِنَّمَا أَطْعَمَهُ اللَّهُ وَسَقَاهُ  

“We broke our fast on a cloudy day at the time of the Messenger of Allah ﷺ, then the sun appeared.” I (one of the narrators) said to Hisham: "Were they commanded to make up for that day?" He said: "It had to be made up." (According to Hisham's opinion).  

أَفْطَرْنَا عَلَى عَهْدِ رَسُولِ اللَّهِ ﷺ فِي يَوْمِ غَيْمٍ ثُمَّ طَلَعَتِ الشَّمْسُ قُلْتُ لِهِشَامٍ أُمِرُوا بِالْقَضَاءِ؟ قَالَ لاَ بُدَّ مِنْ ذَلِكَ