Al-Hakim transmitted on the authority of Abu Hurairah, “Whoever breaks his fast during Ramadan due to forgetfulness is not to make up for the day later or perform an expiation.” It is a sound hadith.

وَلِلْحَاكِمِ: { مَنْ أَفْطَرَ فِي رَمَضَانَ نَاسِيًا فَلَا قَضَاءَ عَلَيْهِ وَلَا كَفَّارَةَ } وَهُوَ صَحِيحٌ [1]‏ .‏