Al Bukhari mentions this tradition in his Sahih, "Kitab al Jihad wa al Siyar": Sahl ibn Sad said: "The Prophet (pbuh&hp) said on the day of (the victory of) Khaybar:

"Tomorrow I will give the standard to a man, by whose hand God shall conquer (Khaybar). He loves God and His Messenger, and God and His Messenger love him."

The people passed the night wondering as to who will receive it and everyone was hopeful of getting it. (The next day) the Prophet (pbuh&hp) declared: "Where is Ali?" He was told: 'He is suffering with an eye pain.' (When Ali came) the Prophet applied his saliva to his eyes and prayed for him. Ali recovered as if he had no pain before. Then the Prophet (pbuh&hp) gave it (the standard) to him...
Refernces:
Muslim in his Sahih, "Kitab al-jihad wa al Siyar" and "Kitab fada'il al Sahabah";
al Tirmidhi in his Sahih, i, 218;
Ibn Majah in Sunan (Matba`at al Faruqi, Delhi) "bab fada'il ashab Rasul Allah (S)";
al Hakim in Mustadrak, iii, 38, 437;
Imam Ahmad ibn Hanbal in Musnad, i, 99, 133, 185, 320, iv, 51, v, 353;
Abu Nu`aym in Hilyat al 'awliya', i, 26, 62;
al Nasa'i in Khasa'is, 4, 5, 7, 8, 32;
al Muttaqi in Kanz al Ummal, v, 283, 285, vi, 394, 395, 405;
al Haythami in Majma al Zawa'id, vi, 150, 151, ix, 119, 123, 124;
Ibn Hajar, Tahdhib al Tahdhib, vii, 337, 339;
al Muhibb al Tabari, al Riyad al Nadirah, ii, 185, 187, 203;
al Tabari, Tarikh, ii, 300;
Ibn Sa`d, al Tabaqat, ii, part one, 80;
Ibn Abd al Barr, al Istiab (Hyderabad, 1336), ii, 450;
al Bayhaqi in Sunan, vi, 362.

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