Sunnah.com reference : Book 2, Hadith 330English translation : Book 2, Hadith 418Arabic reference : Book 2, Hadith 423
Narrated 'Ubai bin Ka'b (RA):
Allah's Messenger (ﷺ) said: "A man's prayer offered with another man is purer than his prayer which he offers alone, and his prayer with two men is purer than his prayer with one and if they are more (in number), it is more beloved to Allah, the Mighty the Majestic."

[Reported by Abu Dawud and an-Nasa'i. And Ibn Hibban graded it Sahih (authentic)].
وَعَنْ أُبَيِّ بْنِ كَعْبٍ ‏- رضى الله عنه ‏- قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ صَلَاةُ اَلرَّجُلِ مَعَ اَلرَّجُلِ أَزْكَى مِنْ صَلَاتِهِ وَحْدَهُ, وَصَلَاتُهُ مَعَ اَلرَّجُلَيْنِ أَزْكَى مِنْ صَلَاتِهِ مَعَ اَلرَّجُلِ, وَمَا كَانَ أَكْثَرَ فَهُوَ أَحَبُّ إِلَى اَللَّهِ عَزَّ وَجَلَّ }  رَوَاهُ أَبُو دَاوُدَ, وَالنَّسَائِيُّ, وَصَحَّحَهُ اِبْنُ حِبَّان َ  1‏ .‏‏1 ‏- حسن.‏ رواه أبو داود (554)‏، والنسائي (2/104‏-105)‏، وابن حبان (2056)‏.‏