كتاب الصوم
Chapter: Those who can fast with difficulty have to feed a poor person
بَابُ: {وَعَلَى الَّذِينَ يُطِيقُونَهُ فِدْيَةٌ}
Sahih al-Bukhari 1949

Narrated Nafi`:

Ibn `Umar recited the verse: "They had a choice either to fast or to feed a poor person for every day, and said that the order of this Verse was canceled.

حَدَّثَنَا عَيَّاشٌ، حَدَّثَنَا عَبْدُ الأَعْلَى، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَرَأَ فِدْيَةٌ طَعَامُ مَسَاكِينَ‏.‏ قَالَ هِيَ مَنْسُوخَةٌ‏.‏
Reference : Sahih al-Bukhari 1949
In-book reference : Book 30, Hadith 56
USC-MSA web (English) reference : Vol. 3, Book 31, Hadith 170
  (deprecated numbering scheme)