Musnad Ahmad 31:26

Muhammad ibn Abi ‘Adi narrated to us, from Shu‘bah, from al-‘Ala’ ibn ‘Abd al-Rahman, from his father, that he heard Abu Sa‘id being asked about the lower garment (izar). He said: "You have fallen upon one who is well-informed. I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The lower garment of the believer is to the middle of the shins. There is no blame—or no sin—upon him in what is between that and the ankles. Whatever is below that is in the Fire. Allah will not look at the one who drags his lower garment out of arrogance.'"