Musannaf Ibn Abi Shayba 16:988

Abu Bakr narrated to us, he said: Zayd ibn al-Hubab narrated to us, he said: Ubayd Allah ibn Abd Allah Abu al-Munib al-Ataki informed us, he said: Abd Allah ibn Buraydah narrated to us, from his father, from the Messenger of Allah (may Allah bless him and grant him peace), that he forbade two types of clothing and two types of sitting. As for the two types of clothing: that you pray in trousers while there is nothing else upon you, and that a man prays in a single garment without wrapping it around himself. And the two types of sitting: that one sits in a squatting position (ihtiba') with a single garment such that his private part is seen, and that one sits between the shade and the sun.