Musannaf Ibn Abi Shayba 3:5519

Waki' narrated to us, he said: Ibn Abi Dhi'b informed us, from Sa'id al-Maqburi, from his father, who said: I said to Abu Hurayrah: "How should I do with my sandals when I pray?" He said: "Place them between your feet, and do not harm any Muslim with them."