Musannaf Ibn Abi Shayba 33:663

Al-Fadl ibn Dukayn narrated to us, he said: Hanash ibn al-Harith narrated to us, he said: Ali ibn Mudrik narrated to us, that Alqamah used to say to al-Aswad: "Why do you punish this body?" He would say: "I only intend rest for it."