Musnad Ahmad 31:924

'Affan narrated to us, Shu'bah narrated to us, Al-'Ala' ibn 'Abd al-Rahman narrated to me, he said: I heard my father narrating, he said: I asked Abu Sa'id about the lower garment (izar), and he said: You have fallen upon one who is informed. The Messenger of Allah (peace and blessings of Allah be upon him) said: "The lower garment of the believer is to the middle of the calf, and there is no harm, or no blame, in what is between it and the ankles. Whatever is below the ankles is in the Fire. And whoever drags his lower garment out of arrogance, Allah will not look at him."