Musnad Ahmad 31:43

Sufyan narrated to us, from al-Ala’ ibn Abd al-Rahman ibn Ya’qub, from his father, who said: I asked Abu Sa’id: Did you hear anything from the Messenger of Allah, may Allah bless him and grant him peace, regarding the lower garment (izar)? He said: Yes, with knowledge I heard him say: "The lower garment of the believer is to the middle of his shins. There is no blame upon him for what is between that and the ankles. And whatever is below the ankles is in the Fire." He said it three times.