Sahih Bukhari Volume 8, Book 78, Hadith Number 697.

Narated By Ziyad bin Jubair : I was with Ibn ‘Umar when a man asked him, “I have vowed to fast every Tuesday or Wednesday throughout my life and if the day of my fasting coincided with the day of Nahr (the first day of ‘Id-al-Adha), (What shall I do?)” Ibn ‘Umar said, “Allah has ordered the vows to be fulfilled, and we are forbidden to fast on the day of Nahr.” The man repeated his question and Ibn ‘Umar repeated his former answer, adding nothing more.