مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 2030
Fasting
Completing what has been neglected - Section 1
عَنْ عَائِشَةَ قَالَتْ: كَانَ يَكُونُ عَلَيَّ الصَّوْمُ مِنْ رَمَضَانَ فَمَا أَسْتَطِيعُ أَنْ أَقْضِيَ إِلَّا فِي شَعْبَانَ. قَالَ يَحْيَى بْنُ سَعِيدٍ: تَعْنِي الشّغل من النَّبِي أَو بِالنَّبِيِّ صلى الله عَلَيْهِ وَسلم
‘A'isha said that when she had some part of the fast of Ramadan to make up she would be able to do so only in Sha'ban. Yahya b. Sa'id said she meant this was due to her being kept from it by her duty to the Prophet. (Bukhari and Muslim.)
Mishkat al-Masabih, 2030مُتَّفق عَلَيْهِ (الألباني)