Summary:

Praying for a believer has many benefits, i.e., angels supplicate for him the same when he prays for his brother who is absent from him, as well as it’s the quickest way to get one’s own prayers answered when he prays for an absent believer. See, for example, some Ahadith:

عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ النَّوْفَلِيِّ عَنِ السَّكُونِيِّ عَنْ أَبِي عَبْدِ اللَّهِ ( عليه السلام ) قَالَ قَالَ النَّبِيُّ ( صلى الله عليه وآله ) لَيْسَ شَيْ‏ءٌ أَسْرَعَ إِجَابَةً مِنْ دَعْوَةِ غَائِبٍ لِغَائِبٍ

Ali Bin Ibrahim, from his father, from Al Nowfaly, from Sakuny, from:

Abu Abdullah^-asws has narrated that Rasool Allah^-saww said: ‘There is nothing quicker to an Answer than a supplication of an absentee for an absentee’.^[1]

وَ قَالَ النَّبِيُّ ص‏ أَسْرَعُ الدُّعَاءِ إِجَابَةً دُعَاءُ غَائِبٍ لِغَائِبٍ.

And the Prophet^-saww said: ‘The supplication of swiftest Response is a supplication of an absentee for an absentee’’.^[2]

وَ رَوَى الْفُضَيْلُ بْنُ يَسَارٍ عَنْ أَبِي جَعْفَرٍ ع قَالَ: أَوْسَعُ دَعْوَةٍ وَ أَسْرَعُ إِجَابَةٍ دَعْوَةُ الْمُؤْمِنِ لِأَخِيهِ بِظَهْرِ الْغَيْبِ

And it is reported by Al Fuzeyl Bin Yasaar,

‘From Abu Ja’far^-asws having said: ‘The vastest of supplication and swiftest in Response is a supplication of the Momin for his brother in the apparent absence.^[3]

وَ رَوَى جَابِرٌ عَنْ أَبِي جَعْفَرٍ ع‏ فِي قَوْلِهِ تَعَالَى‏ وَ يَسْتَجِيبُ الَّذِينَ آمَنُوا وَ عَمِلُوا الصَّالِحاتِ وَ يَزِيدُهُمْ مِنْ فَضْلِهِ‏ قَالَ هُوَ الْمُؤْمِنُ يَدْعُو لِأَخِيهِ بَظْهرِ الْغَيبِ فَيَقُولُ لَهُ الْمَلَكُ وَ لَكَ مِثْلُ مَا سَأَلْتَ وَ قَدْ أُعْطِيْتَ لِحُبِّكَ إِيَّاهُ

And it is reported by Jabir,

‘From Abu Ja’far^-asws regarding Words of the Exalted: And He Answers those who believe and do righteous deeds, and Increases them from His Grace; [42:26]. He^-asws said: ‘He is the Momin supplicating for his brother in the apparent absence, so the Angels says to him: ‘And for you is similar to what you have asked for, and you have been Granted due to your love for him’ (for the sake of Allah^-azwj).^[4]

عُدَّةُ الدَّاعِي، رُوِيَ‏ أَنَّ اللَّهَ تَعَالَى قَالَ لِمُوسَى ادْعُنِي عَلَى لِسَانٍ لَمْ تَعْصِنِي بِهِ

(The book) ‘Uddat Al Daie’ –

‘It is reported that Allah^-azwj the Exalted Said to Musa^-as: “Supplicate to Me^-azwj by a tongue which you^-as have not disobeyed Me^-azwj with it!

فَقَالَ يَا رَبِّ أَنَّى لِي بِذَلِكَ

He^-saww said: ‘O Lord^-azwj! How can that be for me^-as?’

فَقَالَ ادْعُنِي عَلَى لِسَانِ غَيْرِكَ‏

He^-azwj Said: “Supplicate to Me^-azwj upon the tongue of others!’^[5]

And it is reported by Jabir,

‘From Abu Ja’far^-asws regarding Words of the Exalted: And He Answers those who believe and do righteous deeds, and Increases them from His Grace; [42:26]. The Imam^-asws said: ‘He is the Momin supplicating for his brother in the apparent absence, so the Angels say to him: ‘And for you is similar to what you have asked for, and you have been Granted due to your love for him (for the sake of Allah^-azwj).^[6]

[1] Al Kafi V 2 – The Book Of Supplication CH 31 H 7

[2] Bihar Al-Anwaar V 90 – The Book of Zikr (Chapters on supplication) – Ch 26 H 19 b

[3] Bihar Al-Anwaar V 90 – The Book of Zikr (Chapters on supplication) – Ch 26 H 19 c

[4] Bihar Al-Anwaar V 90 – The Book of Zikr (Chapters on supplication) – Ch 26 H 19 f

[5] Bihar Al-Anwaar V 90 – The Book of Zikr (Chapters on supplication) – Ch 22 H 23

[6] Bihar Al-Anwaar V 90 – The Book of Zikr (Chapters on supplication) – Ch 26 H 19