Musannaf Ibn Abi Shayba 3:2968

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from al-A’mash, from Ibrahim, who said: They used to say: Whoever misses a rak'ah of Jumu'ah, let him pray another rak'ah along with it. And whoever does not catch [any of it], let him pray four.