"The Messenger of Allah said: 'If any one of you were to sit on a live coal until it burns his garment, that would be better for him than sitting on a grave."  

قَالَ رَسُولُ اللَّهِ ﷺ لأَنْ يَجْلِسَ أَحَدُكُمْ عَلَى جَمْرَةٍ حَتَّى تَحْرِقَ ثِيَابَهُ خَيْرٌ لَهُ مِنْ أَنْ يَجْلِسَ عَلَى قَبْرٍ  

"do not sit on graves."  

عَنْ رَسُولِ اللَّهِ ﷺ قَالَ لاَ تَقْعُدُوا عَلَى الْقُبُورِ