"There is no fast for the one who did not make it incumbent upon himself from the night before."  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ صِيَامَ لِمَنْ لَمْ يَفْرِضْهُ مِنَ اللَّيْلِ  

“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.”  

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