Musnad Ahmad 31:407

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, he said: I heard al-‘Ala’ ibn ‘Abd al-Rahman narrating from his father, he said: I asked Abu Sa‘id al-Khudri about the lower garment (izar). 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 Muslim is to the middle of the calf, and there is no harm or no blame in what is between it and the two ankles. Whatever is lower than that is in the Fire. Whoever drags his lower garment out of arrogance, Allah will not look at him."