Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: Whoever catches the sermon, Jumu'ah will be two rak'ahs for him; and whoever does not catch it should pray four rak'ahs. And whoever does not catch the ruku' (bowing) should not count the rak'ah by the prostration until he catches the ruku'.
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir as a mawquf narration, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 3164