Mishkat al-Masabih 1066
Ubayy b. Ka‘b said:
God’s Messenger led us in the dawn prayer one day, and when he gave the salutation he asked, “Is so and so present?” and was told that he was not. He asked, “Is so and so present?” and was told that he was not. He then said, “These two prayers* are the ones which are most burdensome to hypocrites. If you knew the blessings they contain you would come to them, even though you had to crawl on your knees. The first row is like that of the angels, and if you knew the nature of its excellence you would race to join it. A man’s prayer said along with another is purer than his prayer said alone, and his prayer with two men is purer than his prayer with one, but if there are more it is more pleasing to God.” 

*i.e. the evening prayer and the dawn prayer.

Abu Dawud and Nasa’i transmitted it.
وَعَنْ أُبَيِّ بْنِ كَعْبٍ قَالَ: صَلَّى بِنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَوْمًا الصُّبْحَ فَلَمَّا سَلَّمَ قَالَ: «أَشَاهِدٌ فُلَانٌ؟» قَالُوا: لَا. قَالَ: «أَشَاهِدٌ فُلَانٌ؟» قَالُوا: لَا. قَالَ: «إِنَّ هَاتَيْنِ الصَّلَاتَيْنِ أَثْقَلُ الصَّلَوَاتِ عَلَى الْمُنَافِقِينَ وَلَو تعلمُونَ مَا فيهمَا لأتيتموهما وَلَوْ حَبْوًا عَلَى الرُّكَبِ وَإِنَّ الصَّفَّ الْأَوَّلَ عَلَى مِثْلِ صَفِّ الْمَلَائِكَةِ وَلَوْ عَلِمْتُمْ مَا فضيلته لابتدرتموه وَإِن صَلَاة الرجل من الرَّجُلِ أَزْكَى مِنْ صَلَاتِهِ وَحْدَهُ وَصَلَاتَهُ مَعَ الرَّجُلَيْنِ أَزْكَى مِنْ صَلَاتِهِ مَعَ الرَّجُلِ وَمَا كَثُرَ فَهُوَ أَحَبُّ إِلَى اللَّهِ» . رَوَاهُ أَبُو دَاوُد وَالنَّسَائِيّ