Musnad Ahmad 31:497

Muhammad ibn Ubayd narrated to us, Muhammad—meaning Ibn Ishaq—narrated to us, from al-Ala’ ibn Abd al-Rahman, from his father, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The lower garment of the Muslim is to the middle of the shin. There is no harm in what is between that and the ankle. And whatever is below the ankle is in the Fire."