It is narrated from Sayyiduna Abdullah (may Allah be pleased with him) that he said: "For whoever catches the sermon, the Friday prayer is two rak'ahs; whoever does not catch it should pray four; and whoever does not catch the bowing (ruku') should not rely on (merely) the prostration until he catches the bowing."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9514