Musnad Ahmad 1156:214

Safwan ibn Isa narrated to us, he said: Muhammad ibn Umarah informed us, from Muhammad ibn Ibrahim al-Taymi, he said: A slave-mother (umm walad) of Ibn Abd al-Rahman ibn Awf narrated to me, she said: I was a woman with a long train (hem), and I used to come to the mosque, and I would drag it. So I asked Umm Salamah, I said: I am a woman with a long train, and I come to the mosque, and I drag it over a dirty place, then I drag it over a clean place. Umm Salamah said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If it passes over the dirty place, then it passes over the clean place, then that is a purification for it."