Musnad Ahmad 1156:18

Abdullah ibn Idris narrated to us, he said: Muhammad ibn 'Umarah 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, she said: I used to drag my garment, and I would pass by a filthy place and a clean place. 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."