His (SWT) saying: O Messenger, announce that which has been revealed to you from your Lord, and if you do not, then you have not conveyed His message. (Quran 5:67)
The verse was revealed about Ali ibn Abi Talib (A), and when this verse came down he [The Prophet (PBUH)] took his hand and said: "To whomever I am his master, Ali (A) is his master, O’Allah (SWT) be pleased with whoever follows him and be the enemy of whoever shows enmity towards him". So Umar (RA) came to him and said: "Congratulations o son of Abi Talib! You have become my master and the master of all believing men and women." And this is the saying of ibn Abbas and al-Baraa' bin ‘azb and Muhammad bin Ali.
Ref: Tafsir Fakhr al-Razi, volume 12, page 53, chapter Surat al-Maida aya 67.
From al-Imam Abu Hassan al-Wahidi, in the book named "Bi isbab al-Nuzol" narrated from Abi Sa'eed al-Khdri (RA) he said: "This verse was revealed "O Messenger, announce that which has been revealed to you from your Lord" the day of Ghadir-e-Khum about Ali ibn Abi Talib (untill the end of the narration)...
Ref: al-Fusool al Muhimmah, volume 1, page 42.
Ibn Hatem and Ibn Mardwiyah and Ibn Asakir has related from Ibn Saaed al Khidar that he said: This verse was revealed "O Messenger! Deliver what has been revealed to you from your Lord" to the Prophet (saw) on the day of Ghadir-e-Khum for Ali ibn Abi Taleb (A). Ibn Mardawiyah related from Ibn Maasod that he said: We read on the time of the Messenger of Allah (saw): "O Messenger! Deliver what has been revealed to you from your Lord THAT ALI IS the chief of the believers and if you do it not, then you have not delivered His message, and Allah will protect you from the people".
Ref: Fathul Qadir, by Mohammad bin Ali bin Mohammad al-Shawkani, volume 2, page 86.
The sunni schoolars of tafsir have confirmed that THIS VERSE DESCENDED FOR IMAM ALI(as) at GHADIR KHUM.
Ref: Tafseer Mazhari, Volume 3 page 353 published by Daar ul Isha`t Karachi
Sunni scholar Abu Naeem Isfahani in his esteemed work Hilayath al Awliya, Volume 1, Page 66-67, records this Hadeeth on the authority of the Sahabi Abu Burdah:
"'There is no doubt that Allah (swt) has made a promise to me with regards to 'Ali. I asked my Lord 'What is it?' Allah (swt) said 'Listen', to which I replied: 'I am listening'. Verily Allah (swt) said 'Ali is the Flag of Guidance, Imam of the Saints, and the Light of Truth for those that follow me, and is that Kalima that has been made COMPULSARY UPON BELIEVERS. Whoever loves him, loves me, whoever angers 'Ali, angers Me, O Muhammad convey this good news to 'Ali…"
Ref: Hilayath al Awliya, Volume 1, Page 66 & 67