Sunan an-Nasa'i 2322
It was narrated that 'Aishah said:
"The Messenger of Allah came to me one day and said: 'Do you have anything (to eat)?' I said: 'No.' he said: 'Then I am fasting.' Then he came to me after that day, and I had been given some Hais. I had kept some for him as he liked Hais. She said: 'O Messenger of Allah, we have been given some Hais and I kept some for you.' He said: 'Bring it here. I started the day fasting.' Then he ate some of it, then he said: 'The likeness of a voluntary fast is that of a man who allocated some of his wealth to give in charity; if he wishes he may go ahead and give it, and if he wishes he may keep it."'
أَخْبَرَنَا عَمْرُو بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا عَاصِمُ بْنُ يُوسُفَ، قَالَ حَدَّثَنَا أَبُو الأَحْوَصِ، عَنْ طَلْحَةَ بْنِ يَحْيَى بْنِ طَلْحَةَ، عَنْ مُجَاهِدٍ، عَنْ عَائِشَةَ، قَالَتْ دَخَلَ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم يَوْمًا فَقَالَ ‏"‏ هَلْ عِنْدَكُمْ شَىْءٌ ‏"‏ ‏.‏ فَقُلْتُ لاَ ‏.‏ قَالَ ‏"‏ فَإِنِّي صَائِمٌ ‏"‏ ‏.‏ ثُمَّ مَرَّ بِي بَعْدَ ذَلِكَ الْيَوْمِ وَقَدْ أُهْدِيَ إِلَىَّ حَيْسٌ فَخَبَأْتُ لَهُ مِنْهُ وَكَانَ يُحِبُّ الْحَيْسَ قَالَتْ يَا رَسُولَ اللَّهِ إِنَّهُ أُهْدِيَ لَنَا حَيْسٌ فَخَبَأْتُ لَكَ مِنْهُ ‏.‏ قَالَ ‏"‏ أَدْنِيهِ أَمَا إِنِّي قَدْ أَصْبَحْتُ وَأَنَا صَائِمٌ ‏"‏ ‏.‏ فَأَكَلَ مِنْهُ ثُمَّ قَالَ ‏"‏ إِنَّمَا مَثَلُ صَوْمِ الْمُتَطَوِّعِ مَثَلُ الرَّجُلِ يُخْرِجُ مِنْ مَالِهِ الصَّدَقَةَ فَإِنْ شَاءَ أَمْضَاهَا وَإِنْ شَاءَ حَبَسَهَا ‏"‏ ‏.‏