Yahya related to me from Malik that Ibn Shihab used to say, "Someone who catches a raka of the jumua prayer should pray another onewith it." Ibn Shihab said, "That is the sunna."Malik said,"I saw the people of knowledge in our city doing that. That is becausethe Messenger of Allah, may Allah bless him and grant him peace, said,'Whoever catches a raka of the prayer has caught the prayer.' "Malik said, concerning some one who was in a crowd on the day ofjumua and did the ruku but was not able to go into sajda until theimam had risen or finished his prayer, "If he is able to do the sajdaand has already done the ruku then he should do the sajda when thepeople stand up. If he is unable to do thesajda until after the imamhas finished the prayer, then I prefer that he begins the prayeragainand does the four rakas of dhuhr."
USC-MSA web (English) reference: Book 5, Hadith 12
Arabic reference: Book 5, Hadith 237