If anyone obtains a rak'ah in the prayer (along with the imam), he has obtained the whole prayer.  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ أَدْرَكَ رَكْعَةً مِنَ الصَّلاَةِ فَقَدْ أَدْرَكَ الصَّلاَةَ  

Yahya related to me from Malik from Ibn Shihab from Abu Salama ibn Abdar-Rahman from Abu Hurayra that the Messenger of Allah ﷺ said "Whoever catches a raka of the prayer has caught the prayer."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ أَدْرَكَ رَكْعَةً مِنَ الصَّلاَةِ فَقَدْ أَدْرَكَ الصَّلاَةَ  

Abu Hurairah reported the Messenger of Allah ﷺ as saying; when you come to pray while we are prostrating ourselves, you must prostrate yourselves, and do not reckon it anything (rak’ah) he has been present at the prayer. (Using translation from Abū Dāʾūd 893)  

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا جِئْتُمْ إِلَى الصَّلَاةِ وَنَحْنُ سُجُودٌ فَاسْجُدُوا وَلَا تَعُدُّوهَا شَيْئًا وَمَنْ أَدْرَكَ الرَّكْعَةَ فَقَدْ أَدْرَكَ الصَّلَاةَ»  

Abu Hurairah reported the Messenger of Allah ﷺ as saying; when you come to pray while we are prostrating ourselves, you must prostrate yourselves, and do not reckon it anything (rak’ah) he has been present at the prayer. (Using translation from Abū Dāʾūd 893)  

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا جِئْتُمْ وَنَحْنُ سُجُودٌ فَاسْجُدُوا وَلَا تَعُدُّوهَا شَيْئًا وَمَنْ أَدْرَكَ رَكْعَةً فَقَدْ أَدْرَكَ الصَّلَاةَ»  

Abu Hurairah reported the Messenger of Allah ﷺ as saying; when you come to pray while we are prostrating ourselves, you must prostrate yourselves, and do not reckon it anything (rak’ah) he has been present at the prayer. (Using translation from Abū Dāʾūd 893)   

"إذَا جِئتُم الصَّلاة ونحنُ سجُودٌ فاسْجُدُوا، ولا تَعُدُّوها شَيئا، وَمنْ أدْرَك الركَعةَ فقد أدْرَكَ الصَّلاةَ".  

"مَنْ أَدْرَكَ رَكْعَةً مِن الصَّلاةِ، فَقَدْ أدْرَكَ الصَّلاةَ".