كتاب الصيام
13  
The Book of Fasting
(25)
Chapter: The Saying of Allah Most High: "... And as for those who can fast with difficulty, they have) a choice either to fast or) to feed a Miskin (poor person) (for every day)" is Abrogated by His saying: "... So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts) that month"...
(25)
باب بَيَانِ نَسْخِ قَوْلِهِ تَعَالَى ‏{‏ وَعَلَى الَّذِينَ يُطِيقُونَه} فِدْيَةٌ بِقَوْلِهِ فَمَنْ شَهِدَ مِنْكُمْ الشَّهْرَ فَلْيَصُمْهُ
Sahih Muslim 1145 a

Salama b. Akwa' (Allah be pleased with him) reported that when this verse was revealed:

"And as for those who can fast (but do not) expiation is the feeding of a needy person" (ii. 183), (he who liked to observe fast did observe it) and he who felt reluctant to observe it ate and expiated till the verse was revealed which abrogated it.
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا بَكْرٌ، - يَعْنِي ابْنَ مُضَرَ - عَنْ عَمْرِو بْنِ الْحَارِثِ، عَنْ بُكَيْرٍ، عَنْ يَزِيدَ، مَوْلَى سَلَمَةَ عَنْ سَلَمَةَ بْنِ الأَكْوَعِ، - رضى الله عنه - قَالَ لَمَّا نَزَلَتْ هَذِهِ الآيَةُ ‏{‏ وَعَلَى الَّذِينَ يُطِيقُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ‏}‏ كَانَ مَنْ أَرَادَ أَنْ يُفْطِرَ وَيَفْتَدِيَ ‏.‏ حَتَّى نَزَلَتِ الآيَةُ الَّتِي بَعْدَهَا فَنَسَخَتْهَا ‏.‏
Reference : Sahih Muslim 1145a
In-book reference : Book 13, Hadith 191
USC-MSA web (English) reference : Book 6, Hadith 2547
  (deprecated numbering scheme)