Musannaf Ibn Abi Shayba 3:2959

Hushaym narrated to us, he said: Isma'il ibn Salim narrated to us, he said: I heard al-Sha'bi saying: Whoever catches the sermon, then it is Jumu'ah. And whoever catches two rak'ahs, then it is Jumu'ah. And whoever catches one rak'ah, he has caught the Jumu'ah, so let him pray another rak'ah. And whoever does not catch the bowing (ruku'), let him pray four.