The Messenger of Allah ﷺ as saying: It is better that one of you should sit on the live coals which burns his clothing and come in contact with his skin than that he should sit on a grave.  

قَالَ رَسُولُ اللَّهِ ﷺ لأَنْ يَجْلِسَ أَحَدُكُمْ عَلَى جَمْرَةٍ فَتَحْرِقَ ثِيَابَهُ حَتَّى تَخْلُصَ إِلَى جِلْدِهِ خَيْرٌ لَهُ مِنْ أَنْ يَجْلِسَ عَلَى قَبْرٍ  

The Messenger of Allah ﷺ as saying: Do not sit on the graves, and do not pray facing them.  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ تَجْلِسُوا عَلَى الْقُبُورِ وَلاَ تُصَلُّوا إِلَيْهَا