Musnad Abu Ya'la al-Mawsili 160:253

Yahya ibn Ayyub al-Maqabiri narrated to us, Ismail narrated to us, he said: Muhammad narrated to me, from Abd al-Rahman ibn Ya'qub, the freed slave of al-Huraqah, from Abu Hurayrah, that 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 shins, and below that is to what is above the ankles. Whatever is below the ankles, then it is in the Fire."