Musannaf Ibn Abi Shayba 3:2955

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Zakariya ibn Abi Za'idah informed us, from Abu Ishaq, from Abu al-Ahwas, he said: Abdullah said: "Whoever catches a rak'ah of the Friday prayer, let him pray another one with it. And whoever does not catch the bowing, let him pray four."