Maliks Muwatta Book 5, Hadith Number 12.

Section : Concerning Someone Who Catches a Raka of the Jumua Prayer.

Yahya related to me from Malik that Ibn Shihab used to say, “Some one who catches a raka of the jumua prayer should pray another one with it.” Ibn Shihab said, “That is the sunna.”

Malik said, “I saw the people of knowledge in our city doing that. That is because the 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 of jumua and did the ruku but was not able to go into sajda until the imam had risen or finished his prayer, “If he is able to do the sajda and has already done the ruku then he should do the sajda when the people stand up. If he is unable to do the sajda until after the imam has finished the prayer, then I prefer that he begins the prayer again and does the four rakas of dhuhr.”