Musnad Ahmad 30:3617

Sa'id ibn 'Amir narrated to us, from Hisham, from Yahya ibn Abi Kathir, from 'Ikrimah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you prays in a single garment, let him cross its two ends over his shoulders."