Musnad Ahmad 30:2340

'Affan narrated to us, he said: Abu 'Awanah narrated to us, he said: 'Abd al-Malik ibn 'Umayr narrated to us, from a man from Banu al-Harith ibn Ka'b, who said: I was sitting with Abu Hurayrah, when a man came to him and asked him, saying: "O Abu Hurayrah, did you forbid the people from fasting on Friday?" He said: "No, by the life of Allah, except that—by the Lord of this sanctuary, and by the Lord of this sanctuary—I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'None of you should fast on Friday unless it is among days he is already fasting.'" He said: Then another came and said: "O Abu Hurayrah, did you forbid the people from praying in their sandals?" He said: "No, by the life of Allah, except that—by the Lord of this sanctuary, and by the Lord of this sanctuary—I saw the Messenger of Allah, peace and blessings of Allah be upon him, praying toward this station (Maqam), while he had his sandals on, then he turned away while they were still on him."