Why is it forbidden to fast on Eid al-Fitr and Eid al-Adha?

It is forbidden to fast on the two Eids, Eid al-Fitr and Eid al-Adha, due to the prohibition of the Prophet, peace and blessings be upon him, concerning them, and a Muslim is required to obey the Prophet, peace and blessings be upon him. The reason for the prohibition of fasting on these two days is to mark a break in fasting and to show the completion of Ramadan in the case of Eid al-Fitr, and for the sake of performing the rituals and eating from the sacrificial animal in the case of Eid al-Adha.