Sunan an-Nasa'i, Volume 1, Book 10, Hadith 844 (DarusSalam) ⓘ
Ubay bin Ka'b said: "One day the Messenger of Allah prayed Fajr, then he said: 'Did so-and-so attend the prayer? They said: 'No.' He said: '(What about) so-and-so? They said:'No' He said: 'These two prayers. are the most burdensome for the hypocrites. If they knew what (virtue) there is in them, they would come, even if they had to crawl. And the virtue of the first row is like that of the row of the angels. If you knew its virtue, you would compete for it. A man's prayer with another man is greater in reward than his prayer alone. And a man's prayer with two other men is greater in reward than his prayer with one other man; the more people there are, the more beloved that is to Allah, the Mighty and Sublime."'
أَخْبَرَنَا إِسْمَاعِيلُ بْنُ مَسْعُودٍ، قَالَ حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ، عَنْ شُعْبَةَ، عَنْ أَبِي إِسْحَاقَ، أَنَّهُ أَخْبَرَهُمْ عَنْ عَبْدِ اللَّهِ بْنِ أَبِي بَصِيرٍ، عَنْ أَبِيهِ، - قَالَ شُعْبَةُ وَقَالَ أَبُو إِسْحَاقَ وَقَدْ سَمِعْتُهُ مِنْهُ، وَمِنْ أَبِيهِ - قَالَ سَمِعْتُ أُبَىَّ بْنَ كَعْبٍ، يَقُولُ صَلَّى رَسُولُ اللَّهِ صلى الله عليه وسلم يَوْمًا صَلاَةَ الصُّبْحِ فَقَالَ " أَشَهِدَ فُلاَنٌ الصَّلاَةَ " . قَالُوا لاَ . قَالَ " فَفُلاَنٌ " . قَالُوا لاَ . قَالَ " إِنَّ هَاتَيْنِ الصَّلاَتَيْنِ مِنْ أَثْقَلِ الصَّلاَةِ عَلَى الْمُنَافِقِينَ وَلَوْ يَعْلَمُونَ مَا فِيهِمَا لأَتَوْهُمَا وَلَوْ حَبْوًا وَالصَّفُّ الأَوَّلُ عَلَى مِثْلِ صَفِّ الْمَلاَئِكَةِ وَلَوْ تَعْلَمُونَ فَضِيلَتَهُ لاَبْتَدَرْتُمُوهُ وَصَلاَةُ الرَّجُلِ مَعَ الرَّجُلِ أَزْكَى مِنْ صَلاَتِهِ وَحْدَهُ وَصَلاَةُ الرَّجُلِ مَعَ الرَّجُلَيْنِ أَزْكَى مِنْ صَلاَتِهِ مَعَ الرَّجُلِ وَمَا كَانُوا أَكْثَرَ فَهُوَ أَحَبُّ إِلَى اللَّهِ عَزَّ وَجَلَّ " .
also referenced under: Hadith 843 (Reference) & Book 10, Hadith 67 (In-Book)