Musnad Ahmad 30:3424

Yazid narrated to us, Muhammad ibn ‘Amr informed us, from ‘Abd al-Rahman ibn Ya‘qub, the freed slave of al-Huraqah, who said: Abu Hurayrah said: Abu al-Qasim (peace and blessings of Allah be upon him) said: "The lower garment of the believer is from the middle of the shins and below that, up to above the ankles. Whatever is below that is in the Fire."