It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There are three supplications that are not turned back: the supplication of the father, the supplication of the fasting person, and the supplication of the traveller."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6392
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