The Messenger of Allah ﷺ said, "There are three whose supplications are not turned back: the just ruler, the fasting person until he breaks his fast, and the supplication of the oppressed. Allah raises it above the clouds on the Day of Resurrection, and the gates of heaven are opened for it, and He says, 'By My might, I shall surely grant you victory, even if it be after some time.'"
حدثنا علي بن محمد حدثنا وكيع عن سعدان الجهني عن سعد أبي مجاهد الطائي وكان ثقة عن أبي مدلة وكان ثقة
قال رسول الله ﷺ ثلاثة لا ترد دعوتهم الإمام العادل والصائم حتى يفطر ودعوة المظلوم يرفعها الله دون الغمام يوم القيامة وتفتح لها أبواب السماء ويقول بعزتي لأنصرنك ولو بعد حين
Its isnad is hasan. Tirmidhi transmitted it in the chapter on Dua, in the section "Those who mention Allah much go ahead," hadith 3598, from the narration of Saadan through him, and said, hasan. Ibn Khuzayma graded it sahih, hadith 1901, as did Ibn Hibban (Mawarid), hadith 2407, 2408.Abu Mudalla was deemed reliable by Tirmidhi, Ibn Khuzayma, and others, so his hadith does not fall below the rank of hasan.