Musnad Ahmad 30:739

Al-Walid ibn Muslim narrated to us, Al-Awza'i narrated to us, Yahya—meaning Ibn Abi Kathir—narrated to us, from Muhammad ibn Ibrahim al-Taymi, from Ya'qub or Ibn Ya'qub, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The lower garment of the believer is to the muscle of his calves, then to the middle of his calves, then to his ankles. Whatever is below that is in the Fire."