Musannaf Ibn Abi Shayba 3:5523

Affan narrated to us, he said: Hammad bin Salamah narrated to us, he said: Abu Na'amah al-Sa'di informed us, from Abu Nadrah, from Abu Sa'id, he said: While the Messenger of Allah (peace and blessings of Allah be upon him) was praying, he took off his sandals and placed them to his left.