Muwatta Imam Malik Book of Jumu'a, Hadith 12

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 ﷺ 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 thesajda until after the imam has finished the prayer, then I prefer that he begins the prayeragain and does the four rakas of dhuhr."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ كَانَ يَقُولُ مَنْ أَدْرَكَ مِنْ صَلاَةِ الْجُمُعَةِ رَكْعَةً فَلْيُصَلِّ إِلَيْهَا أُخْرَى . قَالَ ابْنُ شِهَابٍ وَهِيَ السُّنَّةُ . قَالَ مَالِكٌ وَعَلَى ذَلِكَ أَدْرَكْتُ أَهْلَ الْعِلْمِ بِبَلَدِنَا وَذَلِكَ أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ " مَنْ أَدْرَكَ مِنَ الصَّلاَةِ رَكْعَةً فَقَدْ أَدْرَكَ الصَّلاَةَ " . قَالَ مَالِكٌ فِي الَّذِي يُصِيبُهُ زِحَامٌ يَوْمَ الْجُمُعَةِ فَيَرْكَعُ وَلاَ يَقْدِرُ عَلَى أَنْ يَسْجُدَ حَتَّى يَقُومَ الإِمَامُ أَوْ يَفْرُغَ الإِمَامُ مِنْ صَلاَتِهِ إِنَّهُ إِنْ قَدَرَ عَلَى أَنْ يَسْجُدَ إِنْ كَانَ قَدْ رَكَعَ فَلْيَسْجُدْ إِذَا قَامَ النَّاسُ وَإِنْ لَمْ يَقْدِرْ عَلَى أَنْ يَسْجُدَ حَتَّى يَفْرُغَ الإِمَامُ مِنْ صَلاَتِهِ فَإِنَّهُ أَحَبُّ إِلَىَّ أَنْ يَبْتَدِئَ صَلاَتَهُ ظُهْرًا أَرْبَعًا .