Musnad Ahmad 30:3059

Waki' narrated to us, he said: Shu'bah narrated to us, from Muhammad ibn Ziyad, from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you puts on his sandals, let him begin with the right, and if he takes them off, let him begin with the left. Let him put them both on together, or let him take them both off together."