Musannaf Ibn Abi Shayba 16:591

Abu Bakr narrated to us, he said: Ya'la ibn 'Ubayd narrated to us, from Muhammad ibn Ishaq, from al-'Ala' ibn 'Abd al-Rahman ibn Ya'qub, from his father, from Abu Sa'id, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The lower garment of the believer is to the middle of the calf. Whatever is to the ankle, there is no harm in it. And whatever is below the ankle, it is in the Fire."