Musnad Ahmad 30:349

Yazid ibn Harun narrated to us, Hisham informed us, from Yahya ibn Abi Kathir, Muhammad ibn Ibrahim ibn al-Harith narrated to us, Ya'qub narrated to me, that he heard Abu Hurairah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whatever is below the two ankles of the lower garment is in the Fire." Al-Khaffaf narrated it to us, from Abu Ya'qub.