It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) explained "kal-muhl" as being like the dregs of olive oil; when it is brought near to any inhabitant of Hell, the skin of his face will be scorched.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11672
Hadith Gradingحکم دارالسلام:إسناده ضعيف لضعف دراج فى روايته عن أبى الهيثم