question #3 Sahih Muslim 1139, Book 13, Hadith 181 : How should one resolve a conflict between fulfilling a vow and adhering to Islamic prohibitions?

I had taken a vow that I would fast on the day (but it accidentally) synchronises with the day of Adha or the day of Fitr. Thereupon Ibn ‘Umar (Allah be pleased with him) said: Allah, the Exalted, has commanded fulfilling of the vow, but the Messenger of Allah (ﷺ) has forbidden the observance of fast on this day.