Sunan an-Nasa'i 2323
It was narrated that 'Aishah said:
"The Messenger of Allah passed by my door. He said: 'Do you have anything (to eat)?' I said 'I do not have anything.' He said 'Then I am fasting.' Then he passed by my door a second time and we had been given some Hais. I brought it to him and he ate, and I was surprised. I said: 'O Messenger of Allah, you were fasting, then you ate Hais.' He said: 'Yes, O 'Aishah. The one who observes a fast other than in Ramadan, or making up a missed Ramadan, fast, is like a man who allocated some of is wealth to give in charity; if he wishes he may go ahead and give it, and if he wishes he may keep it."'
أَخْبَرَنَا أَبُو دَاوُدَ، قَالَ حَدَّثَنَا يَزِيدُ، أَنْبَأَنَا شَرِيكٌ، عَنْ طَلْحَةَ بْنِ يَحْيَى بْنِ طَلْحَةَ، عَنْ مُجَاهِدٍ، عَنْ عَائِشَةَ، قَالَتْ دَارَ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم دَوْرَةً قَالَ ‏"‏ أَعِنْدَكِ شَىْءٌ ‏"‏ ‏.‏ قَالَتْ لَيْسَ عِنْدِي شَىْءٌ ‏.‏ قَالَ ‏"‏ فَأَنَا صَائِمٌ ‏"‏ ‏.‏ قَالَتْ ثُمَّ دَارَ عَلَىَّ الثَّانِيَةَ وَقَدْ أُهْدِيَ لَنَا حَيْسٌ فَجِئْتُ بِهِ فَأَكَلَ فَعَجِبْتُ مِنْهُ فَقُلْتُ يَا رَسُولَ اللَّهِ دَخَلْتَ عَلَىَّ وَأَنْتَ صَائِمٌ ثُمَّ أَكَلْتَ حَيْسًا ‏.‏ قَالَ ‏"‏ نَعَمْ يَا عَائِشَةُ إِنَّمَا مَنْزِلَةُ مَنْ صَامَ فِي غَيْرِ رَمَضَانَ - أَوْ غَيْرِ قَضَاءِ رَمَضَانَ أَوْ فِي التَّطَوُّعِ - بِمَنْزِلَةِ رَجُلٍ أَخْرَجَ صَدَقَةَ مَالِهِ فَجَادَ مِنْهَا بِمَا شَاءَ فَأَمْضَاهُ وَبَخِلَ مِنْهَا بِمَا بَقِيَ فَأَمْسَكَهُ ‏"‏ ‏.‏