Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated from the Messenger of Allah (peace and blessings be upon him), he (peace be upon him) said: "There are two supplications that are accepted for the servant: the supplication of the oppressed and the supplication of a Muslim for his (Muslim) brother in his absence."
Hadith Referenceمسند اسحاق بن راهويه / كتاب الدعوات / 837
Hadith Takhrij«مسلم ، كتاب الذكر والدعاء ، باب فضل الدعاء للمسلمين الخ ، رقم : 2732 . سنن ابوداود ، رقم : 1534 . سنن ترمذي ، رقم : 1980 . سنن ابن ماجه ، رقم : 2895 .»