Musannaf Ibn Abi Shayba 3:2972

Abu Bakr narrated to us, he said: Ali ibn Mushir narrated to us, from Sa’id, from Qatadah, from Sa’id ibn al-Musayyib, Anas, and al-Hasan, they said: "If one catches a rak'ah of the Jumu'ah (prayer), he adds another to it. And if he catches them while they are sitting, he prays four."