Musnad Abu Ya'la al-Mawsili 155:203

Ishaq ibn Abi Isra'il narrated to us, Ibn al-Mubarak and 'Abbad ibn al-'Awwam narrated to us, from Hisham, from Muhammad, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade that a man should pray while being *mukhtasiran* (placing his hand upon his waist).