Ala' bin Abd al-Rahman narrates from his father: I asked Abu Sa'id al-Khudri (may Allah be pleased with him) about the lower garment (izar), and he said: I will tell you what I heard from the Messenger of Allah (peace and blessings be upon him): "The lower garment of the believer reaches the middle of the shins. There is no harm in keeping it down to the ankles, but whatever is below the ankles will burn in Hell. And Allah Almighty will not look (with mercy, on the Day of Resurrection) at the one who lets his lower garment trail out of arrogance and pride."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3317