Musnad Ahmad 27:2

Hashim narrated to us, Yunus informed us, from Ziyad ibn Jubayr, who said: I saw a man who came to Ibn Umar and asked him. He said: "He had made a vow to fast every Wednesday, and it happened that it fell on a day of Adha (Sacrifice) or Fitr (Breaking of the Fast)." Ibn Umar said: "Allah has commanded the fulfillment of vows, and the Messenger of Allah, peace and blessings of Allah be upon him, has forbidden us from fasting on the day of sacrifice."