كتاب الصيام
13  
The Book of Fasting
(26)
Chapter: It is Permissible to delay making up missed Ramadan Fasts until before the next Ramadan comes, and this is for those who broke the fast for a reason such as sickness, travel, menstruation and the like
(26)
باب قَضَاءِ رَمَضَانَ فِي شَعْبَانَ ‏
Sahih Muslim 1146 c

In another version of the previous hadith, the words are:

" Yahya said: I think it was due to the regard for the Messenger of Allah (ﷺ)."
وَحَدَّثَنِيهِ مُحَمَّدُ بْنُ رَافِعٍ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا ابْنُ جُرَيْجٍ، حَدَّثَنِي يَحْيَى، بْنُ سَعِيدٍ بِهَذَا الإِسْنَادِ وَقَالَ فَظَنَنْتُ أَنَّ ذَلِكَ لِمَكَانِهَا مِنَ النَّبِيِّ صلى الله عليه وسلم ‏.‏ يَحْيَى يَقُولُهُ ‏.‏
Reference : Sahih Muslim 1146c
In-book reference : Book 13, Hadith 195
USC-MSA web (English) reference : Book 6, Hadith 2550
  (deprecated numbering scheme)