The Messenger of Allah ﷺ said, "There are three whose supplications are never rejected: 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. Allah says, 'By My might, I shall surely grant you victory, even if after some time.'"
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.
Ibn Majah (1752), Tirmidhi (3598), and Ahmad (10183) narrated it, and Ibn Khuzayma (1901) and Ibn Hibban (3428) graded it sahih, all of them from the hadith of Sadan Juhani, from Saad Abu Mujahid Tai, from Abu Madla, from Abu Hurairah, and he mentioned it. Its chain is hasan because of Abu Madla, as explained in the book on fasting. Ibn Hajar also graded it hasan in Amali Adhkar, which Ibn Allan quoted from him in Sharh Adhkar (4/338).This hadith has also come from Abu Hurairah through another route with the wording, "Three supplications are answered without doubt, the supplication of the wronged, the supplication of the parent, and the supplication of the traveler." Abu Dawud (1536), Tirmidhi (1905, 3448), Ibn Majah (3862), Tabarani in Duaa (1314), Ahmad (7510), and Ibn Hibban (2699) narrated it, all of them through routes from Hisham Dastawai, from Yahya bin Abu Kathir, from Abu Jafar the muezzin, that he heard Abu Hurairah say what he mentioned. No one narrated from Abu Jafar the muezzin except Yahya bin Abu Kathir, and Hafiz said in Taqrib, "Acceptable," meaning when corroborated, and he has indeed been corroborated in some parts of the hadith.