Sahih Muslim | Book 6 | Hadith Number 2537

Narrated by Ziyad b. Jubair
Ziyad b. Jubair reported that a person came to Ibn 'Umar (Allah be pleased with them) and said: 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 (may peace be upon him) has forbidden the observance of fast on this day.