Musnad Ahmad 30:2196

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Sa'id ibn Abi Sa'id al-Maqburi narrating from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whatever is below the ankles is in the Fire," meaning the lower garment (izar).