Musannaf Ibn Abi Shayba 3:5489

Waki' narrated to us, he said: Shu'bah narrated to us, from al-Hakam, from 'Abd al-Rahman ibn Abi Layla, he said: The Messenger of Allah (may Allah bless him and grant him peace) prayed in his sandals, so the people prayed in their sandals. Then he took them off, so they took theirs off. When he finished praying, he said: "Whoever wishes to pray in his sandals, let him pray, and whoever wishes to take them off, let him take them off."