In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
The late Mufti of Saudi Arabia Imaam Abdul Azeez Bin Baaz [may Allaah have mercy upon him] said:
Shrines are graves and the Prophetic tradition regarding this affair is that graves are to be raised a hand span from the ground, so that they are recognised as graves and not degraded. This is why in the hadeeth of Sa’d Bin Abee Waqqaas [radiyallaahu-anhu] it is reported that the grave of the Prophet [sallal laahu alayhi wasallam] was raised a hand span and Sa’d also gave orders that the same should be done to his grave. As for building over them, taking them as Masaajid and building domes, this is not permissible. According to the people of knowledge, this is evil, innovation in religion and one of the means to Shirk. And due to this the Prophet [sallal laahu alayhi wasallam] stated in an authentic hadeeth: “The curse of Allaah is upon the Jews and Christians [i.e. those Jews who deviated from the pure message of Moosaa and those Christians who deviated from the pure message of the Messiah], because they took the graves of their Prophets as places of worship”. And in a hadeeth in Saheeh Muslim, the Prophet [sallal laahu alayhi wasallam] forbade that graves should be plastered (made into permanent structures), used as sitting places and building over them. Therefore, it is not permissible to build over graves – neither mosques nor domes, nor other than them, and also they are not to be plastered, because indeed this is one of the means to Shirk – one of the means, because it becomes exalted, invoked besides Allaah and help sought from it, and thus shirk occurs. Building over graves- placing domes, mosques and lamps over them are from the means to Shirk, and this is why the Prophet [sallal laahu alayhi wasallam] warned against it and cursed those who do so.
It is obligated on the Muslims to be warned against this and that they do not build over a grave – neither a Masjid nor other than it, neither a dome nor plastered, neither place lamps on them nor shrouded. All this is not permissible, rather it is from the means to Shirk…. [Ref 1]
Where Was The Grave of The Prophet [sallal laahu alayhi wasallam] Initially?
Shaikh Abdul Waahid Abu Khadeejah [hafidhahullaah] stated as follows:
Proofs against the Sufis who take graves as places of worship―Those who worship Allāh at the graves of righteous men―and those who ask the dead for help. The Green Dome over the Prophet’s Mosque and grave (Kitāb at-Tawhīd, Chapter 20)
Ref 1: https://binbaz.org.sa/fatwas/16337/%D8%AD%D9%83%D9%85-%D8%A8%D9%86%D8%A7%D8%A1-%D8%A7%D9%84%D8%A7%D8%B6%D8%B1%D8%AD%D8%A9-%D8%B9%D9%84%D9%89-%D8%A7%D9%84%D9%82%D8%A8%D9%88%D8%B1 paraphrased