And it is narrated from Al-Hasan, may Allah be pleased with him, that he said: A man of medium height, dark-skinned, from Banu Tamim, named Shu'ayb bin Salih, with four thousand men, wearing white clothes and black banners, will be the leader. No one will face him except that they will be defeated. (From Al-Fatawa Al-Hadithiyyah by Ibn Hajar Al-Haythami)

And it is narrated: When Imran was at the point of death, he called for his brother Amr and said to him: O Amr, I am dying, and this land will be destroyed, and its people will be scattered. It has two blessings and two curses from Allah upon it. As for the first blessing, it is this blessing you are in. And the first curse: this dam will be breached and flood over you, destroying you, your crops, your gardens, and your wealth, and you will scatter across the land. And the second curse: the Abyssinians will overcome you. And the second blessing: Allah will send the Prophet Muhammad, peace and blessings be upon him, with mercy, and the idolaters will be overcome in the end times by the people of religions. They will expel them from the Sacred Mosque and destroy it. Then Allah will send a man from Himyar, called Shu'ayb bin Salih, who will conquer them and expel them from it. And there will be no faith left in the land of Yemen. (From Muluk Himyar by Wahb bin Munabbih)

And it is narrated: ... And the second is when the House of Allah is destroyed, Allah will send a man called Shu'ayb bin Salih, who will conquer those who destroyed it and expel them, until there is no faith left in the world except in the land of Yemen. (From Fath Al-Bari by Ibn Hajar Al-Asqalani)

From these two texts, all we can reliably take is that Shu'ayb was from Himyar in Yemen, or born there, or both. As for a man emerging at the time of the destruction of the Kaaba, the narrations are not sound. But Allah knows best. It is possible that another man with the same name will emerge to fight the Abyssinians at that time.

From the foregoing, we can say the following: The black banners that emerge from Khurasan are two banners. The first emerged during the Abbasid Caliphate, and the other will emerge during the time of the Imam Mahdi, peace be upon him, and he will support him. Shu'ayb bin Salih will be at its forefront, and their clothes will be white, and their turbans will be black, and their number will be four or five thousand, or between them. And these banners will fight the companions of the Sufyani and defeat them. Several places are mentioned where he might have several battles, whether with the army of the Sufyani or others. As for the rest of the battles, the Mahdi will have an army with a black banner, and Shu'ayb bin Salih might be its commander. Shu'ayb will have a great role in the battles during the time of the Imam Mahdi, peace be upon him, or in his company, peace be upon him.

And it is said, in my opinion: That Shu'ayb is the pure soul who will be killed in Medina. It is also said that he is the one who will be whitewashed or the victorious one who will be in the company of the Imam Mahdi when he takes revenge between Mecca and Medina before the pledge of allegiance. If he is indeed the bearer of the Mahdi's banner, then he cannot be the pure soul or the victorious one who is killed before the pledge of allegiance. And Allah knows best.