Hakim bin Abi Harrah said that he heard a man asking Abdullah bin Umar (may Allah be pleased with them both) about vowed fasts: a man names a day and vows that such-and-such a day will not come except that he is fasting on it, and that day turns out to be the day of Eid al-Adha or Eid al-Fitr. So Ibn Umar (may Allah be pleased with them both) said: "Indeed in the Messenger of Allah you have an excellent example" [Al-Ahzab: 21]; the Messenger of Allah (peace and blessings be upon him) did not fast on the Day of Adha or the Day of Fitr, nor did he order fasting on them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8250