Musnad Ahmad 32:1661

'Affan narrated to us, Yazid bin Zuray' narrated to us, Humayd narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The lower garment is to the middle of the shin." When he saw the difficulty of that for the Muslims, he said: "To the ankles; there is no good in what is below that."