The Messenger of Allah ﷺ said: “If one of you were to sit on a live coal that burns him, that would be better for him than if he were to sit on a grave.” (Using translation from Ibn Mājah 1566)  

قَالَ رَسُولُ اللهِ ﷺ «لَأَنْ يَجْلِسَ أَحَدُكُمْ عَلَى جَمْرَةٍ حَتَّى تَحْرِقَ ثِيَابَهُ خَيْرٌ لَهُ مِنْ أَنْ يَجْلِسَ عَلَى قَبْرٍ»  

"do not sit on graves." (Using translation from Nasāʾī 2045)  

عَنْ رَسُولِ اللهِ ﷺ قَالَ «لَا تَقْعُدُوا عَلَى الْقُبُورِ»