Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: Whoever catches one rak'ah of Jumu'ah should join another to it, and whoever misses both rak'ahs should pray four rak'ahs.
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 3171