Musnad Ahmad 31:38

Hashim narrated to us, Laith narrated to us, Ibn Shihab narrated to me, from Ubayd Allah ibn Abd Allah ibn Utbah, from Abu Sa’id, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the *ishtimal al-samma’* (wrapping oneself in a garment such that no opening is left for the arms), and that a man should sit in the *ihtiba’* position (sitting on the buttocks with the knees drawn up to the chest) while wearing a single garment, with nothing of it covering his private parts."