Why Ali (may Allah be pleased with him) was given the title Abu Turab

It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he said: When I had put my foot in the stirrup intending to go to Iraq, Sayyiduna Abdullah bin Salam (may Allah be pleased with him) came to me and said: "Do not go to Iraq, for if you go there, you will meet the edge of the sword." Sayyiduna Ali (may Allah be pleased with him) said: "By Allah! The Messenger of Allah (peace and blessings be upon him) said this to me before you did." Abu al-Aswad says: I said to myself: Subhanallah! I have never seen anything like today: a fighting man telling people things like this (about his own martyrdom).
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

It is narrated from Sayyiduna Ammar bin Yasir (may Allah be pleased with him) that Ali (may Allah be pleased with him) and I were companions to each other in the expedition of Dhat al-Ushayrah. When the Messenger of Allah (peace and blessings be upon him) arrived there and stayed, we saw some people of Banu Mudlij working at a spring of theirs in a palm grove. Ali (may Allah be pleased with him) said to me: "O Abu al-Yaqzan! Would you like us to go to these people and see how they work?" So we went to them and watched their work for a while, then sleep overcame us. Ali (may Allah be pleased with him) and I went off and lay down on the dry dust in a cluster of palm trees and fell asleep. By Allah! No one woke us except the Messenger of Allah (peace and blessings be upon him); he (peace and blessings be upon him) was moving us with his blessed foot, and we had become covered with that dust. Seeing the dust, the Messenger of Allah (peace and blessings be upon him) said: "O Abu Turab!" Then he (peace and blessings be upon him) said: "Shall I not tell you both about the two most wretched of people?" We said: Certainly, O Messenger of Allah! He (peace and blessings be upon him) said: "One is that reddish man of the people of Thamud who hamstrung the she-camel, and the other (O Ali!) is the man who will strike you here, that is, on this part of the head, until this, that is, your beard, is soaked with it (the blood)."
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it with this addition; they only agreed upon the hadith of Abu Hazim from Sahl bin Sa'd: 'Qum Aba Turab' (Get up, O Abu Turab!).