Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade fasting the two days: Eid al-Fitr and Eid al-Ad'ha; wearing a garment that is void of openings for hands; sitting on one’s buttocks while holding his both legs pulled back with his arms in case he wore only one garment; and praying after the Fajr and ‘Asr prayers

عن أبي سعيد الخدري رضي الله عنه قال: «نهى رسول الله صلى الله عليه وسلم عن صوم يومين: الفطر والنحر، وعن اشْتِمَالِ الصَّمَّاءِ، وأن يَحْتَبِيَ الرجل في الثوب الواحد، وعن الصلاة بعد الصبح والعصر».
[صحيح] - [متفق عليه]

Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade fasting the two days: Eid al-Fitr and Eid al-Ad'ha; wearing a garment that is void of openings for hands; sitting on one’s buttocks while holding his both legs pulled back with his arms in case he wore only one garment; and praying after the Fajr and ‘Asr prayers.
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

In this Hadīth the Prophet (may Allah's peace and blessings be upon him) forbade fasting the two days, two types of garments, and two prayers. The two days on which fasting is forbidden are Eid al-Fitr and Eid al-Ad'ha, and the wisdom behind the prohibition of fasting these days is that it is not proper to fast on days of feast and delight. The two types of garments are wearing a garment that is void of openings for the hands, and sitting on one’s buttocks while holding his both legs pulled back with his arms in case he wore only one garment, it has been restricted in the narration of Al-Bukhāri: "If there is nothing covering his private parts." The two prayers are any prayer after the Fajr prayer and after the ‘Asr prayer; this is to eliminate any means leading to imitating the disbelievers who prostrate to the sun at the time of its rising and setting. However, it is permissible during this time to perform the obligatory prayer as well as prayers for specific reasons (i.e. entering the mosque and so on).