Musannaf Ibn Abi Shaybah 5459 — Those who said: he prays four if he

Sa'id bin al-Musayyib, Sayyiduna Anas (may Allah be pleased with him) and al-Hasan said: Whoever catches one rak'ah of the Jumu'ah should add one more rak'ah to it, and if he finds the people sitting in the final sitting of the Jumu'ah prayer, he should pray four rak'ahs.