Hadith: These two prayers are the most burdensome of all prayers for the hypocrites. If you knew what they contain, you would come to them, even if you had to crawl on your knees

عن أُبيِّ بن كعبٍ رضي الله عنه قال:
صلَّى بنا رسولُ الله صلى الله عليه وسلم يومًا الصُّبحَ فقال: «أشاهِد فُلان؟» قالوا: لا، قال: «أشاهِدٌ فُلان؟» قالوا: لا، قال: «إنَّ هاتيَنِ الصَّلاتين أثقَلُ الصَّلَواتِ على المُنافقين، ولو تعلمون ما فيهما لأتيتُمُوهما ولو حَبْوًا على الرُّكب، وإن الصفَّ الأوّلَ على مِثلِ صَفِّ الملائكة، ولو عَلِمتُم ما فضيلتُه لابتَدَرتُموهُ، وإنّ صلاةَ الرجل مع الرجل أزكى من صلاتِه وحدَه، وصلاتَه مع الرجلَين أزكى من صلاتِه مع الرجل، وما كَثُرَ فهو أحبُّ إلى الله تعالى».
[صحيح] - [رواه أبو داود والنسائي وأحمد] - [سنن أبي داود: 554]

’Ubayy ibn Ka‘b (may Allah be pleased with him) reported:
The Messenger of Allah (may Allah's peace and blessings be upon him) led us in the Fajr prayer one day and said: "Is so-and-so present?" They replied: 'No.' He said: "Is so-and-so present?" They replied: 'No.' Thereupon, he said: "These two prayers are the most burdensome of all prayers for the hypocrites. If you knew what they contain, you would come to them, even if 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 with another man is purer than his prayer alone, and his prayer with two men is purer than his prayer with one man; the greater the number, the more pleasing it is to Allah Almighty."
[Authentic hadith] - - [Sunan Abu Daoud - 554]

Explanation

The Prophet (may Allah's peace and blessings be upon him) offered the Fajr prayer one day and then asked: Is so-and-so attending this prayer of ours? The Companions replied: No! Then he said: Is so-and-so present? Another person. They said: No! So, the Prophet (may Allah's peace and blessings be upon him) said: Indeed, the Fajr and ‘Ishā’ prayers are the most burdensome of all prayers for the hypocrites, as people are mostly lazy at their time and they have little chance of Riyā’ (show-off), for they are not seen in darkness.
If you, O believers, knew the additional reward and recompense that results from the Fajr and ‘Ishā’ prayers - as the amount of reward depends on the amount of difficulty - you would come to them, even if you had to crawl on your hands and knees.
The first row, in terms of their closeness to the Imām, is like the row of angels in terms of their closeness to Allah Almighty. If the believers knew the merit of the first row, they would race to join it. And indeed, a man's prayer with another man brings more reward and impact than his prayer alone, and his prayer with two men is better than his prayer with one man. A prayer attended by a lot of worshippers is more rewarding and pleasing to Allah.