Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrates: We set out for Khaybar, and Amir was reciting rajaz verses, saying: "By Allah, were it not for Allah we would not have been guided, nor would we have given charity nor prayed. We have never been free of need of Your bounty; so make our feet firm if we meet (the enemy), and send down tranquillity upon us." The Prophet (peace and blessings be upon him) asked: "Who is this?" They said: It is Amir. He (peace and blessings be upon him) said: "May Allah forgive your sins, O Amir." And the Messenger of Allah (peace and blessings be upon him) never prayed for forgiveness for any of his companions except that he was martyred. Sayyiduna Umar (may Allah be pleased with him) said: O Messenger of Allah, if only you had let us benefit from Amir longer. Then when we reached Khaybar, Marhab came out brandishing his sword; he was their king, and he was saying: "Khaybar knows that I am Marhab, fully armed, a seasoned champion, when wars flare up." Amir came out to face him and said: "Khaybar knows that I am Amir, fully armed, a daring champion." They exchanged a blow each. Marhab's sword struck Amir's shield, and Amir went to strike him, but his own sword turned back on him and cut a vein of the forearm, and his life was in it. At this some of the Companions of the Messenger of Allah (peace and blessings be upon him) began to say: Amir's deeds have been lost; he killed himself. So I came to the Messenger of Allah (peace and blessings be upon him) weeping and said: O Messenger of Allah, have Amir's deeds been lost? The Messenger of Allah (peace and blessings be upon him) asked: "Who said this?" I said: Some of your companions. He (peace and blessings be upon him) said: "Whoever said that has spoken falsely; rather, he will have a double reward."
Then the Messenger of Allah (peace and blessings be upon him) sent me to Sayyiduna Ali bin Abi Talib (may Allah be pleased with him). I went to him while his eyes were sore. He (peace and blessings be upon him) had said: "Today I will give the banner to a man who loves Allah and His Messenger, and whom Allah and His Messenger love." So I brought him, leading him by the hand while his eyes were sore, until I brought him to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) applied his saliva to his eye and it was cured, then he gave him the banner. Then Marhab came out and said: "Khaybar knows that I am Marhab, fully armed, a seasoned champion, when wars flare up." Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said: "I am the one whose mother named him Haydar (lion), like a lion of the forests, of fearsome appearance; I repay them in full, measure for measure with the sandarah." Then he struck him, split Marhab's head and killed him, and the conquest of Khaybar came at the hands of Sayyiduna Ali bin Abi Talib (may Allah be pleased with him).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6216