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It was narrated that 'Aishah, the mother of the Believers, said: "The Messenger of Allah (ﷺ) came to me one day and said: 'Do you have anything (to eat) we said: 'No.' he said: 'Then I am fasting."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا وَكِيعٌ، قَالَ حَدَّثَنَا طَلْحَةُ بْنُ يَحْيَى، عَنْ عَمَّتِهِ، عَائِشَةَ بِنْتِ طَلْحَةَ عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ، قَالَتْ دَخَلَ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم ذَاتَ يَوْمٍ فَقَالَ " هَلْ عِنْدَكُمْ شَىْءٌ " . قُلْنَا لاَ . قَالَ " فَإِنِّي صَائِمٌ " .
sunnah.com