The Jamaat (congregation) of the 5 Fardh Salaat is Sunnah Moukkada (compulsory Sunnah). The evidence of this is in the Holy Quran and the Hadith have many exhortations of this. Allah Ta’ala says:
And establish Prayer and pay Zakat (the Alms-due) regularly and kneel down (together) with those who kneel down. Surah Baqarah (2:43)
Leaving the Jamaat without a valid reason is condemnable. However, if there is a reason, which the Shariah deems as valid, then there is no problem in missing Jamaat. We should strive for regularity in this. There is much excellence of Salaat with congregation. A Salaat with Jamaat is 27 times higher in status than Salaat offered alone. The Holy Prophet (Sallallahu alaihi wa sallam) said:
Translation of Hadith: Salaat with congregation (Jamaat) has 27 times the excellence of Salaat offered alone. (Musnad Imam Ahmed, Hadith No. 5456).
This means that if a person offering Salaat alone, offers the same Salaat 27 times, he will get the reward of offering Salaat with congregation. The scholars say that even then he will miss out on the excellence of saying “Aameen” softly by those offering Salaat in the lead of the Imam.
There is reward even for the steps taken toward the Masjid for the sake of offering Salaat with congregation. That Salaat for which one has to walk to the Masjid from a long distance has been declared the Salaat with the most reward. As given in the Hadith:
Translation of Hadith: In Salaat, in terms of reward, the person who comes from a long way off for Salaat is the foremost. (Kanz Ul Ummal, Hadith No. 20227)
In the excellence of offering Salaat with the Imam, it has also been said that the man doing so will get the reward of spending a night in prayer. As It has been narrated on the authority of Hadhrat Abuzar (May Allah be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said:
Translation of Hadith: The person who stands in Salaat in the lead of an Imam to the point that he returns (i.e. finishes the Salaat and returns), for this person a reward of spending 1 night in worship is written. (Sahih Ibn Hibban, Hadith No. 2073)
There is a Hadith about offering Salaat with congregation:
Translation of Hadith: It has been narrated on the authority of Hadhrat Abu Hurairah (May Allah be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said: When a person goes to the Masjid morning and evening, whenever he goes, Allah Ta’ala prepares Rizq (sustenance) for him in Jannah. (Sahih Ibn Hibban, Hadith No. 2073)
On going to the Masjid for Salaat, on every step 1 virtue is written and a sin is forgiven on every step, as narrated on the authority of Hadhrat Abdullah Bin Umar (May Allah be well pleased with him):
Translation of Hadith: The Holy Prophet (Sallallahu alaihi wa sallam) said: For a person who goes to the Masjid at morning for offering Salaat with congregation, then out of every 2 steps, on one step, a sin is forgiven and on the other, a virtue is written. This grant of Allah Ta’ala is only on the man’s coming and going. (Sahih Ibn Hibban, Hadith No. 2075)
For those who go to the Masjid in darkness, there are glad tidings of absolute radiance in Qiyamah, as the Holy Prophet (Sallallahu alaihi wa sallam) has said. This Hadith is recorded in Mustadrak Alas Sahihain, Hadith No. 724)