Musnad Ahmad 30:1255

Abu al-Nadr narrated to us, Abu Ja’far—meaning al-Razi—narrated to us, from Hisham, from Ibn Sirin, from Abu Hurayrah, who said: "The Prophet, may Allah bless him and grant him peace, forbade that any one of us should pray while 'mukhtasiran' (placing his hand upon his waist)."