Benefits of the Hadith: (They used to build a mosque over the grave of a righteous man who died among them . . .)

The Shaykh : Among the benefits of this hadith are:
That shirk (associating partners with Allah) is great, very great, and that is because of the greatness of its means and paths. The origin of a mosque built over a grave is that it is only for the prayer to Allah, and that is the origin. But because it is feared that the owner of the grave will be worshipped, building over his grave became one of the major sins. The emphasis on the means indicates the greatness of what? It indicates the greatness of the goal.
And among the benefits of this hadith is the protection of the Shari'ah (Islamic sacred law) for the aspect of tawhid (the oneness of Allah) with complete protection, such that it blocked all the means that may lead to shirk (associating partners with Allah), including the prohibition of building a mosque over a grave. This is because the Prophet (peace and blessings be upon him and his family) described those who build mosques over graves as the worst of creation.
And among them is that building over graves involves imitating the Jews and the Christians. So this reality in this Ummah (the Muslim community) now is a verification of the saying of the Prophet (peace and blessings be upon him and his family): And on this basis, the one who builds a mosque over a grave is exactly similar to the Jews and the Christians.
And among the benefits of the hadith is the obligation to demolish the mosque built over a grave. The evidence is that this building is one of the major sins, and it is not permissible to approve of these major sins. This is one point.
Second, the Prophet (peace and blessings be upon him and his family) ordered the demolition of the Mosque of Harm, even though it was not built over a grave, but it contained harm to a mosque next to it. So what is a means to shirk (associating partners with Allah), its demolition is even more necessary.
And among them is a matter in which there is disagreement: Is the prayer valid in this mosque that was built over a grave, or is it not valid? There is disagreement among the scholars. Some said that it is valid because the forbidden thing is the building of the mosque. Yes, some said that the prayer is valid because the forbidden thing is the building of the mosque, and it is separate from the prayer. And it is not reported from the Prophet (peace and blessings be upon him) that he prohibited prayer in mosques built over graves. So it is like if a person prays in a usurped place. The stronger position is that if a person prays in a usurped place, his prayer is valid despite the sin. And this is the madhhab (school of jurisprudence) of the three Imams.
The second opinion is that the prayer in it is not valid because it is prohibited by necessity. That is that the prayer in this mosque is a means to the worship of the owner of the grave, so it is prohibited as a means. And if the act is prohibited, then its existence is a contradiction to Allah and His Messenger, which necessitates prevention. The prevention of executing this thing is due to the saying of the Prophet (peace and blessings be upon him): And this opinion is closer to the truth: that it is not permissible, that the prayer in mosques built over graves is haram (forbidden) and not valid. Especially if the one praying is someone whom people look to as an Imam, meaning that they follow him. Here the sin is doubled and the opinion that the prayer is not valid is strengthened.
And among the benefits of this hadith is that evil varies in degree, due to his saying: And it is so, just as good varies in degree. And it necessitates from this that deeds vary in degree, and it necessitates another necessity that workers vary in degree. And this is the truth: that deeds vary, their good and their bad, and that workers vary according to their deeds. And on this basis, we say that faith (iman) increases and decreases, because the deed is from the faith. So if the deed varies, it necessitates the variation of faith. And this is the truth: that faith varies, it increases with obedience and decreases with disobedience. And it also increases with the strength of the observed signs and their weakness. For the person, whenever he observes the signs, he increases in faith in Allah (Mighty and Majestic). For this reason, Ibrahim (peace be upon him) said: And the person observes this in himself, whenever he sees a great sign that is out of the ordinary, he increases in faith without doubt. And we say "out of the ordinary" because the ordinary things may not have a clear effect on the person because they are familiar to him. So the rising of the sun and its setting, without doubt, are from the great signs of Allah, and yet they are familiar to people and do not have that effect. But if an eclipse occurs or other things in the sun or the moon, the person increases in faith. So faith increases with the firm certainty in the heart and with the deeds.