"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."' (Using translation from Nasāʾī 2322)  

دَخَلَ عَلَيَّ رَسُولُ اللهِ ﷺ يَوْمًا فَقَالَ «هَلْ عِنْدَكُمْ شَيْءٌ؟» فَقُلْتُ لَا قَالَ «فَإِنِّي صَائِمٌ» ثُمَّ مَرَّ بِي بَعْدَ ذَلِكَ الْيَوْمِ وَقَدْ أُهْدِيَ لَنَا حَيْسٌ فَخَبَأْتُ لَهُ مِنْهُ وَكَانَ يُحِبُّ الْحَيْسَ قُلْتُ يَا رَسُولَ اللهِ إِنَّهُ أُهْدِيَ لَنَا حَيْسٌ فَخَبَأْتُ لَكَ مِنْهُ قَالَ «أَدْنِيهِ أَمَا إِنِّي قَدْ أَصْبَحْتُ وَأَنَا صَائِمٌ» فَأَكَلَ مِنْهُ ثُمَّ قَالَ «إِنَّمَا مَثَلُ صَوْمِ التَّطَوُّعِ مَثَلُ الرَّجُلِ يُخْرِجُ مِنْ مَالِهِ الصَّدَقَةَ فَإِنْ شَاءَ أَمْضَاهَا وَإِنْ شَاءَ حَبَسَهَا»  

[Machine] The Messenger of Allah entered upon me and said, "Do you have anything?" I said, "No." He said, "I am fasting." She said, "Then he came to me after that day, and we were given hais yesterday, and I hid some for him. He used to like hais." She said, "O Messenger of Allah, we were given hais, so I hid some for you." He said, "Give it to me, as I have become fasting."  

دَخَلَ عَلَيَّ رَسُولُ اللهِ ﷺ فَقَالَ «هَلْ عِنْدَكُمْ شَيْءٌ؟» فَقُلْتُ لَا قَالَ «فَإِنِّي صَائِمٌ» قَالَتْ ثُمَّ مَرَّ بِي بَعْدَ ذَلِكَ الْيَوْمِ وَقَدْ أُهْدِيَ لَنَا حَيْسٌ بِالْأَمْسِ وَقَدْ خَبَّأْتُ لَهُ مِنْهُ وَكَانَ يُحِبُّ الْحَيْسَ قَالَتْ يَا رَسُولَ اللهِ إِنَّهُ أُهْدِيَ لَنَا حَيْسٌ فَخَبَّأْتُ لَكَ مِنْهُ قَالَ «أَدْنِيهِ أَمَا إِنِّي أَصْبَحْتُ وَأَنَا صَائِمٌ»