Yahya related to me from Malik from Muhammad ibn Umara fromMuhammad ibn Ibrahim that the mother of the son of Ibrahim ibn Abd ar-Rahman ibn Awf questioned Umm Salama, the wife of the Prophet, mayAllah bless him and grant him peace, and said, "I am a woman who wearsa long skirt and (sometimes) I walk in dirty places." Umm Salamareplied, "The Messenger of Allah, may Allah bless him and grant himpeace, said, 'What follows (i.e. clean places) purifies it.' "
USC-MSA web (English) reference: Book 2, Hadith 16
Arabic reference: Book 2, Hadith 46