Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) forbade two ways of wearing clothes: ishtimal al-samma, and sitting with the knees drawn up (ihtiba') in a single garment (in such a way that the legs are apart from the stomach) with the private parts left exposed towards the sky.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3207