Musnad Ahmad 27:796

Waki’ narrated to us, Ibn ‘Awn narrated to us, from Ziyad ibn Jubayr, that a man asked Ibn ‘Umar about a man who vowed to fast a day, and that day coincided with the day of Adha or the day of Fitr. Ibn ‘Umar said: "Allah commanded the fulfillment of vows, and the Messenger of Allah, peace and blessings of Allah be upon him, forbade us from fasting this day."