The Holy Prophet (S) said:
لاَ يَجْتَمِعُ أرْبَعُونَ رَجُلاً فِي أمْرٍ وَاحِدٍ إلاّ اسْتَجَابُ اللهُ تَعَالَى لَهُمْ حَتَّى لَوْ دَعَوْا عَلَى جَبَلٍ لأَزَالُوهُ.
No forty men gather together to pray for a single issue, unless the Almighty God accepts their prayer; to the extent that if they pray against a mountain, it will collapse.
When the believers supplicate to Allah in a congregation and ask their exigency in one voice, as one, their du’a’ is more likely to be answered, for in a group of people, there would surely be, a poor, a bereaved, or an ascetic whose prayer may attract God’s Mercy and Forgiveness. Based on what appears in Islamic teachings, Allah may accept the prayer of others too, for the sake of that person, forgive their sins and grant them their exigency. Many traditions have been narrated from the Prophet (S) and Ahl al-Bayt (as) in this regard, some of which are stated here:
Imam Sadiq (as) stated:
مَا اجْتَمَعَ أرْبَعَةٌ قَطُّ عَلَى أمْرٍ وَاحِدٍ إلاّ تَفَرَّقوُا عَنْ إجَابَةٍ.
No four people congregate to pray for a single issue, unless their exigency is granted when they leave each other.[1]
The Holy Prophet (S) said:
لاَ يَجْتَمِعُ أرْبَعُونَ رَجُلاً فِي أمْرٍ وَاحِدٍ إلاّ اسْتَجَابُ اللهُ تَعَالَى لَهُمْ حَتَّى لَوْ دَعَوْا عَلَى جَبَلٍ لأَزَالُوهُ.
No forty men gather together to pray for a single issue, unless the Almighty God accepts their prayer; to the extent that if they pray against a mountain, it will collapse. [2]
The great Islamic scholar, Ibn Fahd Al-Hilli, narrates from “Wasa’il al-Shi’a” in his book “‘Uddat al-Da’i”:
Allah told Jesus through revelation:
يَا عِيسَى! تَقَرَّبْ إلَى المُؤمِنِينَ وَمُرْهُمْ أنْ يَدْعُونِي مَعَكَ.
O Jesus! Join the believers and order them to call upon me with you ! [3]
Imam Sadiq (as) states:
كَانَ أبِي عَلَيْهِ السَّلاَمُ إذَا حَزَنَهُ أمْرٌ جَمَعَ النِّسَاءَ وَالصِّبْيَانَ ثُمَّ دَعَا وَأَمَّنُوا.
My father always acted this way; whenever something made him sorrowful, he gathered women and children, then he prayed and they said Amen!4
Notes:
1. Al-Kafi: 2/487, Bab l-Ijtima’ fi al-do’a, tradition 2; Jami’ ahadith shi’a: 19/ 354.
2. Mustadrak al-Wasa’il: 5/239, bab 36, tradition 5772; jami’ ahadith al-shi’a: 19/354.
3. Wasa’il al-Shi’a: 7/104, bab 38, tradition 8856.
4. Wasa’il al-Shi’a: 7/105, bab 39, tradition 8860.
Source: http://www.al-islam.org