Hakim bin Abi Hurrah al-Aslami heard a man asking Abdullah bin Umar (may Allah be pleased with him) about a man who named a day and said he would fast on it, and that day fell on Eid al-Adha or Eid al-Fitr. Ibn Umar (may Allah be pleased with him) said: "Indeed, in the Messenger of Allah you have an excellent example" [Surah al-Ahzab: 21]. "The Prophet (peace and blessings be upon him) did not fast on Eid al-Fitr or Eid al-Adha, nor did he order fasting on those days."
(b) Imran bin Husayn (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "A vow is not fulfilled in disobedience to Allah, nor in what the son of Adam does not own."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20143
Ziyad bin Jubayr (may Allah have mercy on him) states: I was with Ibn Umar (may Allah be pleased with him) when a man came to him and said: "I have vowed to fast every Tuesday or Wednesday as long as I live; what if this day falls on the Day of Sacrifice (Eid al-Adha)?" Ibn Umar (may Allah be pleased with him) said: Allah has commanded the fulfilment of vows, but fasting on this day has also been forbidden. The narrator says: The man thought that he had not understood, so he repeated his words a second time, and Ibn Umar (may Allah be pleased with him) said: "Allah has commanded the fulfilment of vows, and fasting on this day has been forbidden."
(b) Yunus (may Allah have mercy on him) says: I mentioned this to al-Hasan (may Allah have mercy on him), and he said: "He should fast another day in its place."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20144