Musnad Ahmad 30:1531

Hassan and Ahmad ibn Abd al-Malik narrated to us, they said: Zuhayr narrated to us, from al-A’mash, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "When you dress, and when you perform ablution, then begin with your right sides." And Ahmad said: "With your right sides."