The Prophet of Allah (blessings of Allah be upon him and his family) has narrated that Jibra`il once came to him after Salat al-Zuhr accompanied by 70,000 Angels and said, ‘O Muhammad, your Lord conveys greetings to you and has granted you two gifts which he has not granted to any Prophet before you.’
I said (to Jibra`il), “What are these two gifts?”
He said: “The Witr Salat which has three Raka’at (i.e. two Rak’at prayer of Shaf’a and one Rak’ah prayer of Witr said late after the 8 Rak’at Nafilah of night) and the saying of five (daily) prayers in congregation (Jama’at).”
I said, “O Jibra`il, what reward is there for my Ummah in saying the prayers in congregation (Jama’at)?”
Jibra`il replied, ‘O’ Muhammad:
• When there are two people (saying congregational prayers), then Allah will record the reward for each Rak’at is being equal to 150 Salat;
• When there are three people, then Allah will record the reward for each Rak’at is being equal to 600 Salat;
• When there are four people, then Allah will record the reward for each Rak’at is being equal to 1,200 Salat;
• When there are five people, then Allah will record the reward for each Rak’at is being equal to 2,400 Salat;
• When there are six people, then Allah will record the reward for each Rak’at is being equal to 4,800 Salat;
• When there are seven people, then Allah will record the reward for each Rak’at is being equal to 9,600 Salat;
• When there are eight people, then Allah will record the reward for each Rak’at is being equal to 19,200 Salat;
• When there are nine people, then Allah will record the reward for each Rak’at is being equal to 37,400 Salat;
• When there are ten people, then Allah will record the reward for each Rak’at is being equal to 72,800 Salat.
[O Muhammad] If there were more than ten people attending the congregational prayer then if all the rivers in the Heavens and the Earth were to become ink, and all the trees (are used as) pens and all of the Jinn, human beings along with the Angels together were to write the reward of one Rak’at prayed, they would not be able to do so.
O’ Muhammad! The (first) Takbir (i.e. Takbiratul Ihram) that the believer performs with the imam (of the congregational prayer) is better than performing 60,000 Hajj and ‘Umrah and better than 70,000 times what is in this world. For ever Rak’at that a believer prays in congregation is better than 100,000 dinar (gold coins) given in charity to the poor. For ever Sajdah that the believer performs in the congregational prayer is better than freeing 1,000 slaves.’“
Ref: Mustadrak al-Wasa’il, Volume 1, Page 487
References:
The Holy Qur`an
Islamic Laws, Ayatullah Sayyid ‘Ali al-Husaini as-Sistani
Mizan al-Hikmah, vol.4, Muhammad al-Ray Shahri
Al-‘Urwah al-Wuthqa, vol. 2, Ayatullah Sayyid Muhammad Kazim al-Yazdi