Hazrat Mahdi's (as) banner

Hazrat Mahdi’s (as) banner is the standard of our Prophet (saas):

He will appear with the woolen standard of our Prophet (saas). That standard 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 (saas), but it will be opened when Hazrat Mahdi (as) appears. (Al-Muttaqi al-Hindi, Al-Burhan fi Alamat al-Mahdi Akhir al-zaman, p. 22)

The features of Hazrat Mahdi’s (as) banner are described as follows in other hadiths:

Certain it is that in the End Times one known as the Hazrat Mahdi (as) will emerge from the furthest part of the country of Magrib (West). And help will go 40 miles before him. Hazrat Mahdi’s (as) flags are striped and white and yellow. The great name of Allah is written upon them. No unit under his banner will be defeated. (Ash-Sharani, Mukhtasar Tazkirah al-Qurtubi, p. 438)

“Obedience is to Allah” is written on Hazrat Mahdi’s (as) banner. (Al-Muttaqi al-Hindi, Al-Burhan fi Alamat al-Mahdi Akhir az-Zaman, p. 65)

The hadiths also note the city that Hazrat Mahdi (as) will spiritually conquer with his banner. It is said he will plant his banner in Constantinople, in other words in Istanbul:

During the [spiritual] conquest of Constantinople he [Hazrat Mahdi (as)] will plant a standard when he goes to make ablution for the morning prayer, and the water will divide in two and withdraw from him. He will cross to the other side through this path that opens... (Ibn Hajar al-Haythami, Al-Qawl al-Mukhtasar fi `Alamat al-Mahdi al-Muntadhar, p. 57)