It is mentioned in Tabaqat of Ibne Saad and Sirah Ibne Hisham:
The Messenger of Allah (S) is buried in the room of his own house where he expired in Madinah. After this, Abu Bakr and Umar were also buried beside him.
Afterwards, a green dome was built over his grave1 as can be seen in the picture opposite.
Sunan Nasai, Kitabul Janaiz.
- 1. Tabaqaat Ibne Saad, 2: 292-294. Sirat Ibne Hisham; 4:343.