Musnad Ahmad 31:431

Hajjaj narrated to us, from Ibn Jurayj, who said: Ibn Shihab informed me, from Ubayd Allah ibn Abd Allah, from Abu Sa’id al-Khudri: "That the Prophet, 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 with his knees drawn up to his chest (*ihtiba’*) while wearing a single garment, with nothing of it covering his private parts."