What was narrated concerning making fasting incumbent upon oneself from the night before, and having the choice (of breaking a voluntary fast) during the day

It was narrated that ‘Aishah said:
“The Messenger of Allah ﷺ would enter upon me and say: ‘Do you have anything (any food)?’ If we said: ‘No,’ he would say: ‘Then I am fasting.’ So he would continue fasting, then it we were given some food, he would break his fast.” She said: “Sometimes he would fast and (then) break fast (i.e., combine fasting and breaking fast in one day).” I said: “How is that?” She said: “Like the one who goes out with charity (i.e., something to give in charity),and he gives some away and keeps some.”

حَدَّثَنَا إِسْمَاعِيلُ بْنُ مُوسَى، حَدَّثَنَا شَرِيكٌ، عَنْ طَلْحَةَ بْنِ يَحْيَى، عَنْ مُجَاهِدٍ، عَنْ عَائِشَةَ، قَالَتْ دَخَلَ عَلَىَّ رَسُولُ اللَّهِ ﷺ فَقَالَ " هَلْ عِنْدَكُمْ شَىْءٌ " . فَنَقُولُ لاَ . فَيَقُولُ " إِنِّي صَائِمٌ " . فَيُقِيمُ عَلَى صَوْمِهِ ثُمَّ يُهْدَى لَنَا شَىْءٌ فَيُفْطِرُ . قَالَتْ وَرُبَّمَا صَامَ وَأَفْطَرَ . قُلْتُ كَيْفَ ذَا قَالَتْ إِنَّمَا مَثَلُ هَذَا مَثَلُ الَّذِي يَخْرُجُ بِصَدَقَةٍ فَيُعْطِي بَعْضَهَا وَيُمْسِكُ بَعْضًا .