Praying in a mosque that contains a grave, with the grave behind the worshippers

Question:
There is a mosque that contains a grave, and they claim it is the grave of a Prophet. They pray the five daily prayers and the Friday prayer in it. When we told them that praying in it is not valid, they argued that the grave is behind them, not in front of them. What is the response to that?

Answer:
If a grave is in the mosque, whether it is the grave of a Prophet or the grave of someone else, the Prophet (peace and blessings be upon him) forbade praying at a grave. If the grave is inside the mosque, whether it is behind, in front, to the right, or to the left, the prayer in this mosque is not valid, because this is a mosque built on a grave. However, if the grave is separate from the mosque because there is a barrier between them, such as a street or a wall—a wall for the mosque and a wall for the grave, meaning there are two walls separating them—and the grave is not connected to the mosque but is isolated, and the mosque is outside the grave, then it is different.