Hadith 4:905
Waki’ 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, and it coincided with [the day of] Fitr or Adha. He said: Allah the Exalted commanded the fulfillment of vows, and the Messenger of Allah (may Allah bless him and grant him peace) forbade fasting on this day.
حدثنا وكيع عن بن عون عن زياد بن جبير قال جاء رجل إلى بن عمر فسأله عن رجل نذر يصوم يوما فوافق ذلك فطرا أو أضحى قال أمر الله تعالى بوفاء النذر ونهى رسول الله صلى الله عليه وسلم عن صيام هذا اليوم