It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: It is better for any one of you to sit on a burning coal that burns his clothes and reaches his skin than to sit on the grave of a Muslim.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9048