What is the ruling on plastering graves with mud (tatyin)?

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Plastering graves with mud is permissible. Ahmad hopes there is no harm in it. Al-Hasan and Al-Shafi'i permitted this practice. The evidence for this permissibility includes the narration that Ibn 'Umar used to look after the grave of 'Asim ibn 'Umar and, after the death of his son while he was absent, he returned, was shown the grave, and ordered its repair. Furthermore, there is a narration from Al-Hasan, from 'Abdullah ibn Mas'ud, that the Messenger of Allah, peace be upon him, said: "The deceased continues to hear the Adhan so long as his grave is not plastered with mud (tuyyina) or, as narrated, so long as his grave is not enclosed/wrapped (tutwa)."