Ibn Majah

Benefit:
One meaning of this is that whoever catches one rak‘ah with the congregation is not deprived of the reward of the congregation. The second meaning is that if one rak‘ah is performed within the prescribed time and then the time ends, that prayer is not considered missed (qada). For example, if one rak‘ah of Fajr was performed and then the sun rose, or one rak‘ah of ‘Asr was performed and then the sun set, in this case, one should complete the prayer. However, it is prohibited to delay the prayer to such an extent without a valid excuse.

Takhrij:
«أخرجه النسائي، المواقيت، باب من أدرك ركعة من الصلاة، حديث:558، وابن ماجه، إقامة الصلوات، حديث:1123، والدارقطني:2 /12.»©Explanation:
➊ This hadith proves that whoever catches one rak‘ah (unit of prayer) of Jumu‘ah (Friday prayer) should complete it by adding the second rak‘ah.
It is evident that the person who was able to catch only one rak‘ah has missed the Friday sermon (khutbah), but his Jumu‘ah prayer will still be valid.
This is also the opinion of Imam Shafi‘i and Imam Abu Hanifah rahimahum Allah.
Being present for the Friday sermon (khutbah) is not a necessary condition.
➋ Although the author rahimahullah has declared the chain of this narration to be authentic here, in "al-Talkhis" he has indicated towards its weakness. However, there is no dispute regarding the authenticity of «مَنْ أَدْرَکَ الرَّکْعَۃَ فَقَدْ أَدْرَکَ الصَّلَاۃَ».
Its general wording includes Jumu‘ah as well.
It also indicates that if one catches less than one rak‘ah, his Jumu‘ah prayer is not valid; in that case, he should perform the four rak‘ahs of the Zuhr (noon) prayer.