Ziyad bin Jubair says that I saw a man come to Sayyiduna Ibn Umar (may Allah be pleased with them both) and ask this question: "I have made a vow that I will fast on such-and-such a day. Now, if that day happens to be Eid al-Adha or Eid al-Fitr, what should I do?" Sayyiduna Ibn Umar (may Allah be pleased with them both) replied: "Allah has commanded the fulfillment of vows, and the Prophet (peace and blessings be upon him) has forbidden us from fasting on the Day of Sacrifice (the tenth of Dhul-Hijjah)."