Musannaf Ibn Abi Shayba 3:2167

Waki’ narrated to us, he said: Abu ‘Ubaydah al-Naji narrated to us, from al-Hasan, from Abu Hurayrah, that he said during his illness: "Seat me up, for I have a trust that the Messenger of Allah, may Allah bless him and grant him peace, entrusted to me." He said: "None of you should turn [his head/gaze] during his prayer. If he must do so, then let it be in other than what Allah has made obligatory upon him."