The Prophet (s) said,
“the day of Ghadir Khumm is the best eid of my umma (people) and that is the day God the Almighty ordered to appoint my brother Ali b. Abi Talib (a) as the standard-bearer of my umma; so that, people will be guided by him after I pass away; and that is the day God perfected the religion and completed blessings upon my umma and chose Islam as their religion.”
Shaykh al-Ṣadūq, al-Amālī, p. 125