Musannaf Ibn Abi Shayba 3:918

Abu Bakr narrated to us, he said: Al-Fadl ibn Dukayn narrated to us, he said: Hurayth ibn al-Sa'ib narrated to us, he said: I asked al-Hasan about the time of Dhuhr, and he said: "When the shadow moves away from the length of the object, that is the time when he prays Dhuhr."