Musannaf Ibn Abi Shayba 16:693

Abu Bakr narrated to us, he said: Abdullah ibn Idris narrated to us, from Ibn ‘Ajlan, from Sa’id ibn Abi Sa’id, from Abu Hurayrah, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Let not one of you walk in one sandal, nor in one leather sock. Let him take them both off, or let him walk in both of them."