Musnad Ahmad 30:3232

Bahz narrated to us, he said: Hammam narrated to us, he said: Qatadah narrated to us, from Abd al-Rahman, the freed slave of Umm Burthan, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, who said: "Indeed, Allah, the Almighty and Majestic, prescribed Friday for those who were before us, but they differed concerning it. Then Allah guided us to it, so the people are followers to us: the Jews are tomorrow, and the Christians are the day after tomorrow."