Musannaf Ibn Abi Shayba 3:837

Abu Khalid al-Ahmar narrated to us, from al-A'mash, from 'Umarah, who said: Al-Aswad was nothing but a monk who would lag behind. He would see that he was praying, so when the time for prayer came, he would make [his camel] kneel, even if it were upon stones.