Ali happened to stay behind the Prophet (ﷺ) and (did not join him) during the battle of Khaibar for he was having eye trouble. Then he said, “How could I remain behind Allah’s Messenger (ﷺ)?” So `Ali set out following the Prophet (ﷺ) , When it was the eve of the day in the morning of which Allah helped (the Muslims) to conquer it, Allah’s Messenger (ﷺ) said, “I will give the flag (to a man), or tomorrow a man whom Allah and His Apostle love will take the flag,” or said, “A man who loves Allah and His Apostle; and Allah will grant victory under his leadership.” Suddenly came `Ali whom we did not expect. The people said, “This is `Ali.” Allah’s Messenger (ﷺ) gave him the flag and Allah granted victory under his leadership.

Narrated by Umm Salama

Sahih Bukhari (Volume 5, Book 57, Hadith 52)

The day of Ghadir Khum is the greatest religious festival (Eid) of my nation. It was the day that Allah, exalted is His remembrance, commanded me to appoint my brother, Ali Ibn Abi Talib, as the guiding banner for my nation, by whom they will be rightly guided after me. It was the day in which Allah finalized religion and fulfilled His blessings on my nation and was pleased that Islam be their faith.

Prophet Muhammad (sws)

Bihar al- Anwar, vol. 94, p. 110, Hadith 2. 

(via sange-saboor)

Did you want me to leave the shrouded body of the Messenger of Allah without burial and come out to quarrel for his sovereignty? By Allah! I could not believe anyone would yearn for it, dispute with us Ahlulbayt in it, and make lawful to do what you did (in usurping our rights).I do not see that the Messenger of Allah left any place for any controversial talks, any plea, or any argument for anyone (by his speech) on the day of Ghadir Khum
Imam Ali (as), al-Ihtijaj , vol. 1, p. 74; Bihar al-Anwar, vol. 28, p. 185 (via 14noor)