Musnad Ahmad 30:989

Yahya ibn Adam narrated to us, Sharik narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who attributed it to the Prophet, may Allah bless him and grant him peace, who said: "For one of you to sit upon a burning coal, such that it burns his clothes until it reaches his skin, is better for him than that he should sit upon a grave."