Musnad Abu Ya'la al-Mawsili 209:47

Abu Ma'mar al-Hudhali narrated to us, Abdullah bin Idris narrated to us, Muhammad bin 'Umarah narrated to us, from Muhammad bin Ibrahim, from an Umm Walad (a slave woman who bore a child to her master) of Ibrahim bin 'Abd al-Rahman bin 'Awf, who said: "I used to lengthen my garment, and I would pass by dirty places and clean places. So I entered upon Umm Salamah and asked her about that. She said: 'I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "What comes after it purifies it."'"