Hadith 1766
Abu Hurairah (May Allah be pleased with him) said:
عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : " لأن يجلس أحدكم على جمرة، فتحرق ثيابه، فتخلص على جلده خير له من أن يجلس على قبر" ((رواه مسلم))
The Messenger of Allah (ﷺ) said, "It is much better for one of you to sit on a live coal, which will burn his clothes and get to his skin than to sit on a grave." [Muslim].
In plain words
Through a startling comparison, the Prophet taught how serious it is to sit upon a grave. The point is the dignity owed to the dead, whose resting place is not ordinary ground to lounge upon.
Reference : Riyad as-Salihin 1766 In-book reference : Book 17, Hadith 256