Musannaf Ibn Abi Shayba 3:3017

Hushaym narrated to us, from Ibn ‘Awn and Hajjaj bin Abi ‘Uthman, from Ibn Sirin, that he used to say: If the people meet you on the day of Jumu’ah and they have already prayed, then proceed to the mosque. If you know what the Imam recited, then recite it and pray.