Bulugh al-Maram, Book 5, Hadith 678 (English) ⓘ

'A’isha (RAA) narrated, ‘One day the Messenger of Allah entered my house and said, “ls there anything here (to eat)?" I said, ‘NO,’ He said, “I shall then be fasting (today)." Then he came to us another day and we said to him, 'Someone has offered us some hais as a gift.’ He then said, "Show it to me. I had been fasting since this morning, “and he ate from it ( as it was a voluntary fast and not during the obligatory fasting of Ramadan). Related by Muslim.

وَعَنْ عَائِشَةَ رَضِيَ اَللَّهُ عَنْهَا قَالَتْ: { دَخَلَ عَلَيَّ اَلنَّبِيُّ ‏- صلى الله عليه وسلم ‏-ذَاتَ يَوْمٍ.‏ فَقَالَ: " هَلْ عِنْدَكُمْ شَيْءٌ? " قُلْنَا: لَا.‏ قَالَ: " فَإِنِّي إِذًا صَائِمٌ " ثُمَّ أَتَانَا يَوْمًا آخَرَ, فَقُلْنَا: أُهْدِيَ لَنَا حَيْسٌ, فَقَالَ: " أَرِينِيهِ, فَلَقَدْ أَصْبَحْتُ صَائِمًا " فَأَكَلَ } رَوَاهُ مُسْلِمٌ 1‏.‏ ‏1 ‏- صحيح.‏ رواه مسلم ( 1154 )‏ ( 170 )‏.‏