Ziyad bin Jubayr reported,
A man came to Ibn Umar 
and said, "I vowed to fast on a particular day, but it happened to fall on the day of Adha or Fitr."Ibn Umar replied, "Allah the Exalted has commanded the fulfillment of vows, and the Messenger of Allah ﷺ forbade fasting on this day."
Azami 4495
Sahihعن زياد بن جبير، قال
جاء رجل إلى ابن عمر رضي الله عنهما، فقال: إني نذرت أن أصوم يوما، فوافق يوم أضحى أو فطر. فقال ابن عمر: أمر الله تعالى بوفاء النذر، ونهي رسول الله ﷺ عن صوم هذا اليوم.
Bukhari related it in the Book of Fasting (1994), and Muslim in the Book of Fasting (1139), both through the route of Ibn Awn, from Ziyad bin Jubayr, with the same chain. The wording given is Muslim's.This shows Ibn Umar's hesitation to issue a definitive ruling when the evidences appeared to conflict, out of religious caution. It may also be understood to indicate giving precedence to a prohibition over a command, since fulfilling a vow can be completed on any other day, unlike fasting, which is not permitted on the day of Eid.