The Messenger of Allah (sallallahu alayhi wa sallam) forbade that the graves should be plastered (made into permanent structures), used as sitting places (for the people) or building over them.
عن جابر رضي الله عنه
قال: نهى رسول الله صلى الله عليه وسلم: أن يجصص القبر، وأن يقعد عليه، وأن يبنى عليه
The Messenger of Allah (sallallahu alayhi wa sallam) forbade that the graves should be plastered (made into permanent structures), used as sitting places (for the people) or building over them.
Who passed it on, from Imam an-Nawawi back to the one who was there.
The same words, in another collection.
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