Musannaf Ibn Abi Shayba 7:43

Abu Bakr narrated to us, he said: Waki’ ibn al-Jarrah narrated to us, from Ibn ‘Awn, from Ziyad ibn Jubayr, who said: A man came to Ibn ‘Umar and asked him about a man who vowed to fast a day, but it coincided with the day of Fitr or Adha. Ibn ‘Umar said: Allah commanded the fulfillment of vows, and the Messenger of Allah (may Allah bless him and grant him peace) forbade fasting on this day.