Musannaf Ibn Abi Shayba 3:5378

7755 Malik ibn Isma'il narrated to us, he said: Mandil narrated to us, from al-A'mash, from Mujahid, from 'Ubayd ibn 'Umayr, from Abu Dharr, from the Prophet, may Allah bless him and grant him peace, who said: "The earth has been made for me a purifier and a place of prostration."