Musnad Ahmad 441:1

Hammad ibn Khalid narrated to us, Mu'awiyah ibn Salih narrated to us, from Yunus ibn Sayf, from Ghudayf ibn al-Harith, or al-Harith ibn Ghudayf, who said: Of the things I have forgotten, I have not forgotten that I saw the Messenger of Allah, peace and blessings of Allah be upon him, placing his right hand upon his left in the prayer.