Sahih Muslim
Sahih Muslim 1146 a
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
Arabic hadith text
حَدَّثَنَا أَحْمَدُ بْنُ عَبْدِ اللَّهِ بْنِ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ أَبِي، سَلَمَةَ قَالَ سَمِعْتُ عَائِشَةَ، - رضى الله عنها - تَقُولُكَانَ 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 .
Translation
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).