Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
ahmad-zuhd:574 – ʿAbdullāh > al-Ḥasan b. ʿAbd al-ʿAzīz al-Baṣrī > ʿAbdullāh b. Yaḥyá al-Maʿāfirī > Ḥaywah > Shuraḥbīl b. Sharīk > Abū ʿAbd al-Raḥman al-Ḥubulī / ʿAbdullāh b. Yazīd > al-Ṣunābiḥī > Abū Bakr al-Ṣiddīq

[Machine] That he heard Abu Bakr As-Siddiq say, "Indeed, the prayer of a brother for his brother in Allah is answered."  

الزهد لأحمد:٥٧٤ – حَدَّثَنَا عَبْدُ اللَّهِ حَدَّثَنَا الْحَسَنُ بْنُ عَبْدِ الْعَزِيزِ الْبَصْرِيُّ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يَحْيَى الْمَعَافِرِيُّ حَدَّثَنَا حَيْوَةُ عَنْ شُرَحْبِيلَ بْنِ شَرِيكٍ أَنَّهُ سَمِعَ أَبَا عَبْدِ الرَّحْمَنِ الْحُبُلِيَّ وَهُوَ عَبْدُ اللَّهِ بْنُ يَزِيدَ يَقُولُ حَدَّثَنِي الصُّنَابِحِيّ

أَنَّهُ سَمِعَ أَبَا بَكْرٍ الصِّدِّيقَ يَقُولُ «إِنَّ دُعَاءَ الْأَخِ لِأَخِيهِ فِي اللَّهِ ﷻ يُسْتَجَابُ»  

See similar narrations below:

Collected by Suyūṭī
suyuti:1-96b – al-Ṣnābḥá Nh Smʿ Bā Bkr al-Ṣdyq
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السيوطي:١-٩٦b –

"عَنِ الصنابحى أنه سمع أبا بكر الصديق: أَنَّ دُعَاءَ الأَخِ لأَخِيهِ فِى الله يُسْتَجَابُ".  

[خ] البخاري في الأدب [عم] زيادات عبد الله بن أحمد في زوائد الزهد [هب] البيهقى في شعب الإيمان