Quran 5:55 Imam Ali (as) appointed as successor

Abu Dhar al-Ghifari: Both of my ears may turn deaf and both of my eyes may become blind if I speak a lie. I heard the Messenger of Allah, Allah's blessings and peace be upon him and his posterity, saying, "Ali is the guide of the righteous and the slayer of the infidels. he who has helped him is victorious and he who has abandoned him is forsaken." One day I said my prayers in the company of the Prophet. A beggar came to the mosque and begged for alms, but nobody gave him anything. Ali was in a state of kneeling in the prayer. He pointed out his ring to the beggar, who
appraoched him and removed the ring from his finger. Thereupon the Prophet, Allah's blessings and peace be upon him and his posterity, implored Allah the Mighty and Glorious, saying: "O Allah! My brother Moses begged you saying, 'My Lord, delight my heart and make my task easy and undo the knot in my tongue so that they may understand me, and appoint from my kinsmen, Haroon, my brother, as my vizier, and strenghthen my back with him and make him participate in my mission so that we may glorify You and remember You more frequently. Certainly You see us.' And You inspired him: 'O Musa! All your requests have been granted.' O Allah! I am your slave and your prophet. Delight my heart and make my task easy and appoint from among my kinsmen Ali as my vizier and strengthen my back with him."

Abu Dhar, then, proceeded

By Allah, the Messenger of Allah, Allah's blessings and peace be upon him and his posterity had not yet finished his supplication when the trustworthy Gabriel descended to him with this Verse"

"Certainly Allah is Your Master, and His Prophet and those who believe who establish prayer and give charity while they bow. And whoever takes Allah and His Messenger and those who believe as a guardian, so surely the party of Allah will be victorious."

Sunni reference: Tafsir al-Kabir, by al-Tha'labi under the commentary of verses 5:55-56 of Quran.

Other Sunni references affirming 5:55 revealed for Imam Ali (as):

(1) Tafsir al-Kabir, by Ahmad Ibn Muhammad al-Tha'labi, under Verse 5:55
(2) Tafsir al-Kabir, by Ibn Jarir al-Tabari, v6, pp 186,288-289
(3) Tafsir Jamiul Hukam al-Quran, by Muhammad Ibn Ahmad Qurtubi, v6, p219
(4) Tafsir al-Khazin, v2, p68
(5) Tafsir al-Durr al-Manthoor, by al-Suyuti, v2, pp 293-294
(6) Tafsir al-Kashshaf, by al-Zamakhshari, Egypt 1373, v1, pp 505,649
(7) Asbab al-Nuzool, by Jalaluddin al-Suyuti, Egypt 1382, v1, p73 on the
authority of Ibn Abbas
( Asbab al-Nuzool, by al-Wahidi, on the authority of Ibn Abbas
(9) Sharh al-Tjrid, by Allama Qushji
(10) Ahkam al-Quran, al-Jassas, v2, pp 542-543
(11) Musnad Ahmad Ibn Hanbal, v5, p38
(12) Kanzul Ummal, by al-Muttaqi al-Hindi, v6, p391, Tradition #5991
(13) al-Awsat, by Tabarani, narrated from Ammar Yasir
(14) Ibn Mardawayh, on the authority of Ibn Abbas

If you inquire how this plural word is applicable to Ali, may Allah be gracious to him, who is an individual, I shall say that though this verse is about Ali, an individual, the plural form is used in order to presuade others to act similarly and to give alms as readily as Ali did. There is also an implied instruction that the faithful should keep themselves always on the look out for occasions of acts of sympathy, benevolence and charity to the poor and the needy, and readily do the needful without waiting till the completion of even so important a duty as saying a prayer.
Sunni reference: Tafsir al-Kashshaf, by al-Zamakhshari, v1, p649