Ali happened to stay behind the Prophet (ﷺ) and (did not join him) during the battle of Khaibar for he was having eye trouble. Then he said, “How could I remain behind Allah’s Messenger (ﷺ)?” So `Ali set out following the Prophet (ﷺ) , When it was the eve of the day in the morning of which Allah helped (the Muslims) to conquer it, Allah’s Messenger (ﷺ) said, “I will give the flag (to a man), or tomorrow a man whom Allah and His Apostle love will take the flag,” or said, “A man who loves Allah and His Apostle; and Allah will grant victory under his leadership.” Suddenly came `Ali whom we did not expect. The people said, “This is `Ali.” Allah’s Messenger (ﷺ) gave him the flag and Allah granted victory under his leadership.
Narrated by Umm Salama
Sahih Bukhari (Volume 5, Book 57, Hadith 52)