(a) Abu al-Ahwas and Hubayrah both narrate from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: Whoever catches one rak'ah of the Friday prayer should pray the second rak'ah as well, and whoever misses both rak'ahs should pray four rak'ahs.
(b) Isa bin Yunus narrates from Zakariyya: Whoever finds the people sitting should pray four rak'ahs.
(c) Al-A'mash narrates from Abu Ishaq: When you miss the bowing, pray four rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5740