Why is the grave of the Messenger of Allah, peace and blessings be upon him, and those of his two companions, may Allah be pleased with them, veiled with a curtain and a metal fence, and is this mentioned in Islamic law?

The wisdom behind the walls around the noble graves is to keep the graves outside the Prophet's Mosque, because the Prophet, peace and blessings be upon him, forbade taking graves as mosques. The graves are separated from the mosque by the three surrounding walls. The Prophet, peace and blessings be upon him, and his two companions were buried in Aisha's chamber and were not buried in the mosque. Umar and Uthman, may Allah be pleased with them both, took care during the expansion of the Prophet's Mosque not to include the Prophet's houses, and similarly, in the last expansion, care was taken not to include the left side so that the graves remain adjacent to the mosque but not inside it.