Quran
Allah says: “And remember Allah during the appointed Days”Quran Surah al-Baqarah 2:203 These are the days of Tashreeq. This was the view of Ibn ‘Umar(R) and most of the scholars.

Hadith
Narrated 'Aishah(R): The Messenger of Allah (ﷺ) performed the obligatory circumambulation of the Ka'bah at the end of the day of sacrifice after he had offered the noon prayer. He then returned to Mina and stayed there during the Tashriq days and he threw pebbles at the jamrahs when the sun declined. He threw seven pebbles at each of the jamrahs, uttering the Takbir (Allah is most great) at the time of the throwing the pebble. He stood at the first and the second jamrah, and prolonged his standing there, making supplications with humilation. He threw pebbles at the third jamrah but did not stand there. Sunan Abi Dawud 1973

Fasting on Ayyam E Tashreeq
Uqbah bin Amir (R) said: The Messenger of Allahﷺsaid: "The Day of Arafah, the Day of Nahr, and the Days of Tashriq(11th, 12th, and 13th of Dhul-Hijjah) are Eid for us. The people of Islam, and they are days of eating and drinking,"Jami at-Tirmidhi 773, Sunan Ibn Majah Vol 1: 1720 and Sunan an Nasai 3007



The Prophetﷺforbade fasting on these days: “Do not fast on these days, for they are the days of eating, drinking and remembering Allaah.”Musnad Ahmad, 10286; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 3573