It is narrated from Sayyiduna Abu Bakr Siddiq (may Allah be pleased with him) that the supplication of a religious brother for his religious brother is accepted.
Hadith Referenceالادب المفرد / كتاب / 624
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه ابن وهب فى الجامع : 161 و الدولابي فى الكني : 959/3 و البيهقي فى الشعب : 8640»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: The point is that when two individuals have a relationship and brotherhood due to religion, with no other familial connection, then their supplication (du'a) for one another is accepted. In other words, when their love is for the sake of Allah and there is no worldly interest involved, their supplication for each other is accepted. Thus, among the causes for the acceptance of supplication is also love and relationship for the sake of Allah.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 624