The area between the Sacred Chamber and the pulpit (Mimbar) is known as Riadhul Jannah, that is, the Garden of Paradise. It is currently distinguished by a green carpet and is also called Rawdah.
Abu Hurayra (may Allah be pleased with him) said that the Prophet (SAW) said:
“Between my house and my pulpit lays a garden from the gardens of Paradise, and my chair is located at my fountain (Al-Kauthar).” [Bukhari]
Scholars interpreted the above narrative as follows: The garden is parallel to the garden above it in Paradise. Or is it actually a garden of paradise that will be returned to heaven in the hereafter. Or it looks like paradise gardens, because the peace and tranquility that are felt in it at the remembrance of Allah, remind of paradise. [Akhbar Madinah Tour Rasool, Ibn Najjar]
The original size is approximately 22 meters in length and 15 meters in width, part of it is located in the Prophet’s chamber (SAW).
Mehrab (Prayer Niche):
The Prophet (SAW) prayed about 16 months facing towards Al-Quds in Jerusalem upon his arrival in Medina. If someone had to leave the “Aisha column”, leaving it in the direction of your back, the fifth column will correspond to the door marked Bab-e-Jibraeel. This fifth column was an approximate place of prayer for the Prophet (SAW) for a specified period. It was not far from the extreme northern border of the original mosque.
After a change in the Qiblah, the Prophet (SAW) for a short period of time offered salah near the column of Aisha, referring to the Ka’ba. Later he began to lead the salat at the place where Mehrab Nabwi is now. Please note that at that time the Prophet (SAW) and the four caliphs in this place there was no Mehrab.
In 91 AH, Umar bin Abdul Aziz created a prayer niche in the form of a mehrab. Since then, he has been called Mehrab Nabwi. If you stand in Mehrab for the offer of salah, your place of sajda will be where the Prophet’s feet were (SAW). The place of the prophet (SAW) of the sajda is intentionally covered with the thick wall of Mehrab.
The Prophet (SAW) usually leaned against the trunk of a palm tree during a sermon. Ansar humbly offered him: “If you approve, we can make a chair for you.” The Prophet (SAW) approved it, and the pulpit was made. The Prophet Mohammed (SAW) sat down at the pulpit to make a speech. When the Prophet (SAW) began to use the new pulpit, the old tree yearned for him like a camel without his calf. The Prophet (SAW) hugged the trunk until he calmed down, and then ordered to dig a ditch and decently bury it.
The Prophet (SAW) used to give sermons while standing on third rung. When Abu Bakr (may Allah be pleased with him) became caliph, he stood on the second step, and Umar bin Khattab (may Allah be pleased with him) stood at the first. Khalif Usman bin Affan did what Umar did (may Allah be pleased with him) for six years before he returned to the Prophet’s previous position. Amir Mu’awiyah (may Allah be pleased with him) made a pulpit of nine steps. The leaders began to sit on the seventh step. Since then, the pulpit has been preserved in this form, and since that time Khateeb has been sitting on the seventh step.
Over the centuries, the pulpit has been repeatedly replaced. The real pulpit was established by Sultan Murad in 998 AH.
Sources: History of Madinah Munawwarah – Dr. Muhammad Ilyas Abdul Ghani, Wikipedia History of Madina – Ali Hafiz