Hadith: 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, the Almighty

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]

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 to the hypocrites. If you knew what they contain (of virtue), 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 know 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, the Almighty."
Sahih/Authentic. - [An-Nasaa’i]

Explanation

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.' So-and-so here refers to some of the hypocrites, as reported in the narration of Ad-Dārmi: "They said: 'No,' referring to a group of hypocrites who had not attended the prayer." He said: "These two prayers are the most burdensome of all prayers to the hypocrites." The two prayers, here, refer to the ‘Ishā' and the Fajr, as related in a Hadīth reported by Abu Hurayrah (may Allah be pleased with him) in which the Prophet (may Allah's peace and blessings be upon him) said: "The most burdensome prayer to the hypocrites is the ‘Ishā' prayer and the Fajr prayer." [Al-Bukhāri and Muslim] In principle, all obligatory prayers are burdensome to the hypocrites. Allah, the Almighty, says: {And when they stand for prayer, they stand lazily} [Sūrat An-Nisā': 142]. However, the ‘Ishā' and Fajr prayers are the most burdensome among them. The ‘Ishā' prayer comes at a time of rest and preparation for sleep after a day of labor and tiredness. The Fajr prayer, on the other hand, comes at a time when sleep is most enjoyable. Hence, the Adhān of the Fajr prayer includes this phrase: "Prayer is better than sleep." "If you know what they contain," meaning: the reward and merit resulting from the performance of the ‘Ishā' and Fajr prayers in congregation in the mosque. Indeed, the reward for a deed is commensurate with the difficulty involved therein. "You would come to them, even if you had to crawl on your knees." That is; you would come to the mosques to perform these two prayers along with the congregation of the Muslims, even if you had to crawl upon your hands and knees, like a crawling child, in case something prevented you from walking on foot to the mosque; you would not neglect to obtain the merit of these prayers. "The first row is like that of the angels." The first row is the one right behind the Imām. In this sentence, the Prophet (may Allah's peace and blessings be upon him) likens the proximity of the first row to the Imām to the nearness of the row of angels to Allah, Glorified and Exalted. If you knew the merit that is obtained by offering the prayer in the first row, you would hasten and race to gain this reward. This is similar to another statement made by the Prophet (may Allah's peace and blessings be upon him) when he said: "If the people knew what is in the Adhān and the first row, then if they could not but cast lots to get it, they would do so." "A man's prayer with another man is purer than his prayer alone." In other words, a man's prayer with another man brings greater reward than his prayer alone. "And his prayer with two men is purer than his prayer with one man." If they are three, this is better than two, for the former are more in number. "The greater the number, the more pleasing it is to Allah, the Almighty." The greater the congregation, the more preferable and pleasing it will be in the sight of Allah. This indicates how meritorious the congregational prayer is. Indeed, 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; and the greater the number, the more pleasing it is to Allah, Glorified and Exalted.