Musannaf Ibn Abi Shayba 13:2408

Abu Bakr narrated to us, he said: 'Abbad bin al-'Awwam narrated to us, from Muhammad bin Ishaq, from 'Abdullah bin Abi, the freed slave of Umm Salamah or 'Aishah, he said: I saw Sa'd carrying a basket of human excrement to a land of his called al-Zughabah. So I said to him: "O Abu Ishaq, do you carry this?" He said: "Indeed, a basket of excrement is a basket of grain."