What is the ruling on praying in a mosque that contains a grave?

The questioner : In the name of Allah, the Most Gracious, the Most Merciful. Praise be to Allah, Lord of the worlds, and may Allah send prayers and blessings upon our Prophet Muhammad, and upon his family and all his Companions. May Allah reward you, Honorable Shaykh, and benefit us with your knowledge. We are in a country where there are many graves in the mosques. Is the prayer in a mosque that has a grave valid? And what is the ruling if the grave is on the side of the mosque or in its rear?

The Shaykh : Yes. First, we must know that it is not permissible to build over graves. It is not permissible to build over graves, and building over graves is among the major sins, because the Prophet (peace and blessings be upon him) said at the end of his life . So making mosques over graves is from the characteristics of the Christians and the Jews. If a mosque is built over a grave, it must be demolished, and prayer in it is forbidden, because it was founded on other than the fear of Allah.
For this reason, Allah the Exalted said regarding the Mosque of Harm, which is less severe than the mosque built over graves, saying to His Prophet Muhammad (peace and blessings be upon him): , . So the mosques built over graves, as I told you, are not founded on the fear of Allah, and they must be demolished, and prayer in them is not permissible, because it is forbidden.
As for if the mosque existed before the grave, then the prayer in it is valid on the condition that the grave is not in the qiblah of the mosque. Because if it is in the qiblah of the mosque, it falls under the prohibition of praying toward graves, because it is established from the Prophet (peace and blessings be upon him) that he said: . Did you understand? For the sake of detail, say: detail, my brother.
Well done. If the grave existed first, then building the mosque over it is forbidden, and the prayer in this mosque is invalid, and what must be done? Demolish the mosque. That is correct. Yes.
In the qiblah of the mosque. May Allah bless you. That is correct. But these mosques now, which are said to have graves in them, we need two things. The first is proof that there is a grave in it. It may be said there is a grave, but there is no grave. An example of that is the head of al-Husayn ibn 'Ali ibn Abi Talib (may Allah be pleased with them both). Al-Husayn (may Allah be pleased with him) was killed in a well-known battle. Some historians claimed that his head is in Iraq, and others that his head is in Sham, and others that his head is in Egypt. How many heads does one man have? Glory be to Allah the Great. One man does not have multiple heads. There is no doubt that he (may Allah be pleased with him) is in one of them.
And it is in Iraq without doubt, and it cannot be in Sham or in Egypt. There are some graves that are said to be... what? Yes, some mosques are said to be built over the grave of al-Husayn, or over his head, over the grave in which his head is. So we say first: we must look to see if this grave that is claimed to be in the mosque is correct or not? This is one point. If it is confirmed that it is in it, we ask another question: what is it?
No, my brother, before that: which is earlier? Was the mosque built over the grave, or was the grave placed in the mosque after it was built? If the mosque was built over the grave, then the prayer in it is not valid and it must be demolished. And if it is the opposite, that the mosque existed before the dead person was buried in it, then the prayer in it is valid on the condition that it is not in the qiblah of the mosque, because if it is in the qiblah of the mosque, it has entered into what the Prophet (peace and blessings be upon him) prohibited, where he said: .