Musnad Ahmad 30:2429

Yahya ibn Sa'id narrated to us, from Shu'bah, Muhammad ibn Ziyad narrated to us, from Abu Hurayrah; and Ibn Ja'far narrated to us, from Shu'bah, from Muhammad ibn Ziyad, who said: I heard Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, saying: "Put sandals on both of them together, or take them off both together. If you put on sandals, begin with the right, and if you take them off, begin with the left."