Zahabi, Mizan al-Ehtedal V.3, page 76; Ibn Marduyeh Esfahani, Manaqib, page 67.

And he has quoted a hadith from Anas who said, "I was in the presence of the Messenger of Allah (s.a.w.a.). All of a sudden, I saw Ali coming from the front. The Prophet (s.a.w.a.) said, "Anas!" I replied: Yes, Oh the Messenger of Allah. He said, "The one who is coming from the front is my proof upon my Ummah on the Resurrection Day”. Ibn Demeshqi, Jawaher al-Matalib, V.1, page 193; Ibn Asaker, Tarikh Madinah-Damishq, V.42, page 309; Zahabi, Mizan al-Ehtedal, V.4, page 127; Qunduzi, Yanabi al-Muwadda, V.2, page 170.