It is permissible to observe a voluntary fast with an intention formed during the day before the sun reaches its Zenith, and it is permissible for one who is observing a voluntary fast to break his fast with no excuse, although it is better for him to complete it

Sahih Muslim · Book 13 · Hadith 1154 a

وَحَدَّثَنَا أَبُو كَامِلٍ، فُضَيْلُ بْنُ حُسَيْنٍ حَدَّثَنَا عَبْدُ الْوَاحِدِ بْنُ زِيَادٍ، حَدَّثَنَا طَلْحَةُ، بْنُ يَحْيَى بْنِ عُبَيْدِ اللَّهِ حَدَّثَتْنِي عَائِشَةُ بِنْتُ طَلْحَةَ، عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ، - رضى الله عنها - قَالَتْ قَالَ لِي رَسُولُ اللَّهِ صلى الله عليه وسلم ذَاتَ يَوْمٍ ‏"‏ يَا عَائِشَةُ هَلْ عِنْدَكُمْ شَىْءٌ ‏"‏ ‏.‏ …

'A'isha, the Mother of the Believers (Allah be pleased with her), reported that one day the Messenger of Allah (ﷺ) may peace be upon him) said to me: 'A'isha, have you anything (to eat)? I said: 'Messenger of Allah, ther…