ما يحرم على الصائم
Abu ‘Ubayd, the servant of Ibn Az'har, reported: I attended the Eid (prayer) with ‘Umar ibn al-Khattāb (may Allah be pleased with him) who said: "These are two days which the Messenger of Allah (may Allah's peace and blessings be upon him) prohibited fasting on: the day you break your fast (Eid al-Fitr) and the day you eat from your sacrificial animals (Eid al-Ad'ha)."
عن أبي عبيد، مولى ابن أزهر، قال: شهدت العيد مع عمر بن الخطاب -رضي الله عنه-، فقال: هذان يومان نهى رسول الله -صلى الله عليه وسلم- عن صيامهما: يوم فطركم من صيامكم، واليوم الآخر تأكلون فيه من نُسُكِكُم.
شرح الحديث :
Allah, the Almighty, has appointed two days to be the feasts of Muslims, and each of them is related to a religious ritual. The first day of Eid al-Fitr is connected with the completion of fasting in the month of Ramadan. Hence, it is obligatory upon a Muslim to break his fast on that day out of gratitude to Allah for completing His favor upon him by enabling him to fast, and as a manifestation of the blessing of breaking the fast that Allah has commanded after the fast; Allah, the Almighty, says: {And [wants] for you to complete the period and to glorify Allah for that to which He has guided you; and that you may be grateful} [Sūrat al-Baqarah: 185]. The second is the day of Eid al-Ad'ha which is connected with the ritual of offering sacrificial animals. People slaughter animals and offer them as sacrifice and demonstrate the rituals of Allah, the Almighty, by eating from those animals. Therefore, it is obligatory on Muslims not to fast on those two days, and fasting on this day is prohibited.