Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) forbade "wrapping a single garment around oneself in such a way that the hands are completely enclosed." Some say that ishtimal al-samma is that one wraps the garment around oneself and then lifts it from one side and throws it over the shoulder, which exposes the private parts. And he also forbade "sitting with the knees drawn up (ihtiba') in a single garment when there is nothing over one's private parts."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3206