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 a

Abu Salama reported:

I heard 'A'isha (Allah be pleased with her) as saying: I had to complete some of the fasts of Ramadan, but I could not do it but during the month of Sha'ban due to my duties to the Messenger of Allah (ﷺ) or with the Messenger of Allah (ﷺ).
حَدَّثَنَا أَحْمَدُ بْنُ عَبْدِ اللَّهِ بْنِ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ أَبِي، سَلَمَةَ قَالَ سَمِعْتُ عَائِشَةَ، - رضى الله عنها - تَقُولُ كَانَ يَكُونُ عَلَىَّ الصَّوْمُ مِنْ رَمَضَانَ فَمَا أَسْتَطِيعُ أَنْ أَقْضِيَهُ إِلاَّ فِي شَعْبَانَ الشُّغُلُ مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَوْ بِرَسُولِ اللَّهِ صلى الله عليه وسلم ‏.‏
Reference : Sahih Muslim 1146a
In-book reference : Book 13, Hadith 193
USC-MSA web (English) reference : Book 6, Hadith 2549
  (deprecated numbering scheme)