Musnad Ahmad 441:2

'Abd al-Rahman ibn Mahdi narrated to us, Mu'awiyah narrated to us, from Yunus ibn Sayf, from al-Harith ibn Ghudayf, or Ghudayf ibn al-Harith, who said: Of the things I have not forgotten, I have not forgotten that I saw the Messenger of Allah, may Allah bless him and grant him peace, placing his right hand upon his left in the prayer.