Sahih Adab al-Mufrad

It is narrated from Sayyiduna Abu Bakr Siddiq (may Allah be pleased with him): The supplication of a brother made for the sake of Allah is accepted.

It is narrated from Safwan bin Abdullah bin Safwan. Darda bint Abu Darda was in his marriage. He narrates that (once) I reached them (Abu Darda) in Syria and saw that Umm Darda was at home and Abu Darda was not there. Umm Darda asked me: "Do you intend to perform Hajj this year?" I said: "Yes." She said: "Pray for good for us, the Prophet (ﷺ) used to say: 'The supplication of a Muslim man for his Muslim brother in his absence is accepted. An angel is appointed over his head; when he prays for good for his brother, the angel says: Ameen, and may you have the same.'" Safwan narrated that after this, I met Abu Darda in the market, and he narrated the same. He used to narrate this (hadith) from the Prophet (ﷺ).

Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) narrated: A man said, "O Allah! Forgive me and Muhammad only, and no one else." Upon this, the Prophet (peace and blessings be upon him) said, "You have restricted your supplication to many people."