Musnad Ahmad 31:266

Ya'la 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 believer is to the middle of the calf. There is no harm in what is down to the ankle. And whatever is below the ankle is in the Fire."