He traced it back to the Prophet ﷺ, who said, "For one of you to sit on a burning coal that scorches his clothes until it reaches his skin is better for him than to sit on a grave."
حدثنا يحيى بن آدم حدثنا شريك عن سهيل بن أبي صالح عن أبيه
يرفعه إلى النبي ﷺ قال لأن يجلس أحدكم على جمرة فتحرق ثيابه حتى تفضي إلى جلده خير له من أن يجلس على قبر
Its isnad is sahih, and it will come again by way of Suhayl also, at 9036, 9730, and 10844. Muslim also reported it, 1:265, by way of Jareer, and by way of Darawardi and Thawri, all three of them from Suhayl, with the same wording. Abu Dawud likewise reported it, 3228, and Nasai, 1:287, and Ibn Majah, 1566, all three of them by way of Suhayl.