Musannaf Ibn Abi Shayba 3:2960

Hushaym narrated to us, he said: Yunus informed us from al-Hasan, and al-Mughirah from Ibrahim, and Isma'il bin Abi Khalid from al-Sha'bi; they said: Whoever catches a rak'ah of Jumu'ah, let him add another to it. And whoever does not catch the bowing (ruku'), let him pray four.