Sunnah.com reference : Book 2, Hadith 366English translation : Book 2, Hadith 446Arabic reference : Book 2, Hadith 449
Narrated Ibn 'Umar (RA):
Allah's Messenger (ﷺ) said: "Whoever is in time for a Rak'ah of the Friday prayer, or any other prayer, should add another (Rak'ah) to it and his prayer will then be complete."

[an-Nasa'i, Ibn Majah and ad-Daraqutni reported it; the wording is from ad-Daraqutni. Its chain of narrators is Sahih (authentic), but Abu Hatim strengthened the view that it is Mursal (missing link after the Tabi'i)].
وَعَنِ ابْنِ عُمَرَ قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ مَنْ أَدْرَكَ رَكْعَةً مِنْ صَلَاةِ اَلْجُمُعَةِ وَغَيْرِهَا فَلْيُضِفْ إِلَيْهَا أُخْرَى, وَقَدْ تَمَّتْ صَلَاتُهُ }  رَوَاهُ النَّسَائِيُّ, وَابْنُ مَاجَهْ, وَاَلدَّارَقُطْنِيُّ, وَاللَّفْظُ لَهُ, وَإِسْنَادُهُ صَحِيحٌ, لَكِنْ قَوَّى أَبُو حَاتِمٍ إِرْسَالَهُ .‏  1‏ .‏‏1 ‏- صحيح.‏ رواه النسائي (1/274‏-275)‏، وابن ماجه (1123)‏، والدارقطني (2/12/12)‏.‏