One of the main proofs that the Turkish Nation will fulfill this historic responsibility is the way that in the hadith about the end times our Prophet (may Allah bless him and grant him peace) makes particular reference to Istanbul and to Turkey. As revealed by the Prophet (may Allah bless him and grant him peace), Hazrat Mahdi (pbuh) will be active in Istanbul, will construct the Turkish-Islamic Union by bringing the dispersed Turkic states back together, and will appear with the Sacred Trusts.

HAZRAT MAHDI (PBUH) WILL SPIRITUALLY CAPTURE ISTANBUL

Narrated by ibn 'Amr: … The Prophet (may Allah bless him and grant him peace) said, "O Ummah! Six things will happen before the Hour … The sixth is the conquest of Medina."

He was asked, "O Messenger of Allah, which medina [city]?"
He said, "Constantinople [Istanbul]."
(*) This is the conquest of Constantinople by Hazrat Mahdi (pbuh). (Signs of the Last Day, p. 204; Ramuz al-Ahadith, 296)

Allah will capture Constantinople [Istanbul] through His beloved friends [Hazrat Mahdi (pbuh)]. (Signs of the Last Day, p. 181)

Nations will come under his [Hazrat Mahdi (pbuh)] command. Allah will make him the instrument of the [spiritual] capture of Constantinople [Istanbul]. (Al-Burhan fi Alamat al-Mahdi Akhir al-Zaman, p. 56)

HAZRAT MAHDI (PBUH) WILL SERVE AMONG THE TURKS

Hazrat Mahdi (pbuh) will never leave Rum, the Turks [because Turkey used to be known as the region of Rum]. (Narrated from Ish'afu'r-Ragibin, Tilsimlar, p. 212)

A Tirmidhi hadith says that "the Hour will not come until Hazrat Mahdi (pbuh) rules the Arabs" (Tirmidhi, Fitan: 43). This means he will not appear from among the Arabs because in order to rule over the Arabs he will have not to be one of them. (Signs of the Last Day, p. 170)

Some people will appear from the East and prepare the way for Hazrat Mahdi (pbuh). Hazrat Mahdi (pbuh) will be the ruler among them. (Ibn Majah, Kitabul Fitan: 35 (4088) This hadith shows that he will come from among a people existing in or coming from the east. This refers to the Turks, who were then living in the east and who subsequently settled in Anatolia, and Allah knows the truth. (Signs of the Last Day, p. 171)

Ibn Khaldun and al-Qurtubi record that Hazrat Mahdi (pbuh) will come from the Mashriq (the east), Khorasan (the area to the west of the Caspian Sea) and the Amu Darya (these being regions inhabited by Turks). (Macdonald, Encyclopedia of Islam, 7:478.)
These things all show that Hazrat Mahdi (pbuh) will engage in intensive activities among the Turks. (Saban Dogen, Mehdi ve Deccal [The Mahdi and the Dajjal], p. 172)
Sayyid Ahmad Husamaddin (ra) says this about Hazrat Mahdi's (pbuh) birthplace in his Istihracname:

"An individual will come from among the Muslims, and the honor of that individual will be radiant like the rays of the Sun, shining around from the greatest peak of the Caucasus." (Osman Yuksel Serdengecti, City with No Temple, Serdengecti Publication: VI, p.107)

THE FRAGMENTED TURKISH STATES WILL BE UNITED BY HAZRAT MAHDI (PBUH)

... And dispersed Turkish flags will appear and help my son [Hazrat mahdi (as)] in every nook and cranny. (al-Ghaybah al-Nomani, p. 323)

... Allah will cause him [Hazrat Mahdi (pbuh)] to conquer Rum, Daylam, Sindh, India, Kabul and the Caspian. (Sheikh Muhammad bin Ibrahim Nomani, al-Ghaybah al-Nomani, p. 274)

According to a hadith of the Prophet's (may Allah bless him and grant him peace), Hazrat Mahdi (pbuh) will first appear from Turkey with the Turkish flag and will then become the spiritual ruler of other Islamic countries with their green flags. He will be instrumental in the founding of the Turkish-Islamic Union and will assume the spiritual leadership of it. The hadith is as follows:

In that year the disappearance of my son [Hazrat Mahdi (pbuh)], who will have the red flag and afterwards the green flag, will be announced. (Sheikh Muhammad bin Ibrahim Nomani, al-Ghaybah al-Nomani, p. 170)

HAZRAT MAHDI (PBUH) WILL HAVE THE SACRED TRUSTS WITH HIM

The scholars who narrated the hadith about the end times spoke of the events of the end times being based in the centers of the caliphate of their own times. This is why, with regard to the place where Hazrat Mahdi (pbuh) will appear from, different scholars have cited such places as Iraq, Damascus, Kufa and Medina, which were the centers of the caliphate in their own times. However, the common feature of these accounts is that the events of the end times will take place in the center of the caliphate. The final center of the caliphate was "Istanbul." The caliphate was officially abolished at the beginning of the last century, and it was never transferred anywhere else after that time. Consequently, Istanbul is the only city to still bear this spiritual title. The two standards, sword and robe of our Prophet (may Allah bless him and grant him peace), together with other sacred trusts, are kept in Topkapi Palace, Istanbul.

It is related from Abdullah ibn Shurafa: "The banner of our Prophet (may Allah bless him and grant him peace) will be adorned, together with Hazrat Mahdi (pbuh)." (Al-Burhan fi Alamat al-Mahdi Akhir al-zaman, p. 65)

Nuaym ibn Hammad narrates from Abu Jafar; "Hazrat Mahdi (pbuh) will appear in the evening with the banner, robe, sword, signs, radiance and beautiful expression of our Prophet (may Allah bless him and grant him peace). (Mar'iy Ibn Yusuf Ibn Abi Baqr ibn Ahmad ibn Yusuf al-Maqdi'si Faraidu Fawaidi'l Fikr Fi'l Imam al-Mahdi al-Muntazar)
The Mahdi (pbuh) will appear with the banner of our Prophet (may Allah bless him and grant him peace). That banner is unstitched, black, and has four corners. It has never been unfurled since the death of our Prophet (may Allah bless him and grant him peace) but will be unfurled by Hazrat Mahdi (pbuh). (The Book of Portents of the Awaited Mahdi, pp. 41-42, 52, 54)

As for signs, he [Hazrat Mahdi (pbuh)] will have with him the shirt, the sword, and the banner of our Prophet (may Allah bless him and grant him peace). That banner has never been unfurled since the death of our Prophet (may Allah bless him and grant him peace). And, it will not be unfurled until the appearance of Hazrat Mahdi (pbuh). (Portents of the Last Day, p. 164)

He will appear with the camel hair banner of our Prophet (may Allah bless him and grant him peace). That banner has four corners, is unstitched and black in color. There is a halo on it. It has not been unfurled since the death of our Prophet (may Allah bless him and grant him peace), but it will be opened when Hazrat Mahdi (pbuh) appears. (Al-Burhan fi Alamat al-Mahdi Akhir al-zaman, p. 23)

One important feature of this banner, which we are told will only be unfurled by Hazrat Mahdi (pbuh) in the end times is that "it has never been unfurled since the death of the Prophet (may Allah bless him and grant him peace)." The banner has never been opened by any state, including the Ottoman Empire, out of respect for the Prophet (may Allah bless him and grant him peace), and was never taken out of its cover even during the battles or during the ceremonies it was taken to. The banner, preserved in this way for 14 centuries, will be opened when the moral values of Islam rule the world, with the coming of Hazrat Mahdi (pbuh).