It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There are three people whose supplication is not rejected: 1. the just ruler, 2. the fasting person until he breaks his fast, and 3. the supplication of the oppressed is raised up above the clouds, the gates of the heaven are opened for it, and Allah, the Blessed and Exalted, says: 'By My might! I will surely help you, even if it be after a while.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6393