Mishkat al-Masabih #1938
Fasting
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
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.)
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About Mishkat al-Masabih
This hadith is #1938 from Mishkat al-Masabih, in the Book of Fasting.