Musnad Ahmad 30:3701

'Abd al-Samad narrated to us, Hammad narrated to us, Suhayl narrated to us, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "For one of you to sit upon a burning coal until it burns his clothes and reaches his skin is better for him than to step upon a grave."