كتاب الصيام
5  
Fasting
Bulugh al-Maram 670
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 .‏
Reference : Bulugh al-Maram 670
In-book reference : Book 5, Hadith 670
English translation : Book 5, Hadith 691
Hadith Text
Reference