The Prophet (PBUH&HF) said: "There is amongst you a person who will fight for the interpretation of the Quran just as I fought for its revelation." The people around him raised their heads and cast inquisitive glances at the Prophet (PBUH&HF) and at one another. Abu Bakr and Umar were there. Abu Bakr inquired if he was that person and the Prophet (PBUH&HF) replied in the negative. Then Umar inquired if he was that person and the Prophet (PBUH&HF), replied "No. He is the one who is repairing my shoes (i.e., Ali)."
Abu Said Khudri said: Then we went to Ali and conveyed the good news to him. He did not even raise his head and remained as busy as he was, as if he had already heard it from the Messenger of Allah (PBUH&HF)."
Sunni references:
al-Mustadrak, by al-Hakim, v3, p122, who said this tradition is genuine based on the criteria of al-Bukhari and Muslim.
al-Dhahabi, also records it in his Talkhis al-Mustadrak and admitted that it is genuine according to the standard of the two Shaikhs.
Khasa'is, by al-Nisa'i, p40
Musnad Ahmad Ibn Hanbal, v3, pp 32-33
Kanz al-Ummal, by al-Muttaqi al-Hindi, v6, p155
Majma' al-Zawa'id, by al-Haythami, v9, p133