This raised platform, behind Rawdah Mubarak and in accordance with Bab-e-Jibraeel, is the approximate place where the Prophet (saw) from time to time spread out his prayer mat and performed Salah Tahajud.
Isa bin Abdullah (may Allah be pleased with him) said: “When visitors used to leave at night, the Prophet (saw) used to laid out the prayer mat behind Ali’s (may Allah be pleased with him) hut and usually offered a voluntary salah there, Once one man saw how he offered voluntary salah in this place during the month of Ramadan. This man also began to offer voluntary salah in this place. Another man went this way, and he also began his salaah. A third person followed these two people. Soon there gathered a large number of people. When the Prophet (saw) saw a lot of people, he wrapped his prayer mat and left. When these people met with the Prophet (saw) in the morning, they said: “We only tried to follow you, offering voluntary salah at night.” The Prophet (saw) said: “I was very worried about you. I was worried that Allah might make the offer of a night salat during Ramadan a must for you, and you may not keep up with it. ”
Sources: History of Madinah Munawwarah – Dr. Muhammad Ilyas Abdul Ghani