The Messenger of Allah ﷺ said, "Three whose supplications are never rejected: the fasting person until he breaks his fast, the just ruler, and the supplication of the oppressed, which Allah raises above the clouds and for which He opens the gates of heaven. The Lord says, 'By My might, I will surely grant you victory, even if after some time.'"
حدثنا أبو كريب حدثنا عبد الله بن نمير عن سعدان القمي عن أبي مجاهد عن أبي مدلة
قال رسول الله ﷺ ثلاثة لا ترد دعوتهم الصائم حتى يفطر والإمام العادل ودعوة المظلوم يرفعها الله فوق الغمام ويفتح لها أبواب السماء ويقول الرب وعزتي لأنصرنك ولو بعد حين
Its isnad is hasan. Ibn Majah also narrated it, no. 1752, from the hadith of Saadan Qabi through the same route. Ibn Khuzayma graded it sahih, no. 1901, as did Ibn Hibban, no. 2407, 2408. It has a corroborating witness mentioned earlier, no. 2526.