Musnad Ahmad 32:1748

'Ali ibn Ishaq narrated to us, 'Abdullah—meaning Ibn al-Mubarak—informed us, Humayd narrated to us, from Anas, who said: He said—as if he meant the Prophet, may Allah bless him and grant him peace—said: "The lower garment is to the middle of the shin." It was difficult for them, so he said: "Or to the two ankles, and there is no good in what is below that."