Musnad Abu Ya'la al-Mawsili 209:103

Abu Khaythama narrated to us, Abdullah ibn Idris narrated to us, Muhammad ibn Umara narrated to us, from Muhammad ibn Ibrahim, from an Umm Walad (a slave woman who bore a child for her master) of Ibrahim ibn Abd al-Rahman ibn Awf, who said: I used to drag my garment, and I would pass by a filthy place and a clean place. So I asked Umm Salama about that. She said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "What comes after it purifies it."