Sahih Muslim 971aBook 11, Hadith 124
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا جَرِيرٌ، عَنْ سُهَيْلٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لأَنْ يَجْلِسَ أَحَدُكُمْ عَلَى جَمْرَةٍ فَتُحْرِقَ ثِيَابَهُ فَتَخْلُصَ إِلَى جِلْدِهِ خَيْرٌ لَهُ مِنْ أَنْ يَجْلِسَ عَلَى قَبْرٍ " .
It is better that one of you should sit on live coals which would burn his clothing and come in contact with his skin than that he should sit on a grave.