Musnad Ahmad 30:2774

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from ‘Abd al-Malik ibn ‘Umayr, from a man from Banu al-Harith, that he heard Abu Hurayrah saying: "I am not forbidding you to fast on Friday, but I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'Do not fast on Friday, unless you fast a day before it.'" "And I am not praying in sandals, but I saw the Messenger of Allah, peace and blessings of Allah be upon him, praying in his sandals."