Musnad Ahmad 30:140

Sufyan narrated to us, from al-Zuhri, from Sa’id, from Abu Hurayrah, attributing it to the Prophet, may Allah bless him and grant him peace: "When it is the day of Friday, there are angels at every door of the mosque, writing down the first [to arrive], then the first. So when the Imam comes out, the scrolls are folded."