Hadith: There are three supplications that will undoubtedly be answered: the supplication of the oppressed

عن أبي هريرة رضي الله عنه مرفوعاً: «ثلاثُ دَعواتٍ مُستجابات لا شك فِيهن: دعوةُ المظلومِ، ودعوةُ المسافرِ، ودعوةُ الوالدِ على وَلدِه».
[حسن] - [رواه أبو داود والترمذي وابن ماجه وأحمد]

Abu Hurayrah (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said: "There are three supplications that will undoubtedly be answered: the supplication of the oppressed, the supplication of the traveler, and the supplication of a parent against his child."
Hasan/Sound. - [Ibn Maajah]

Explanation

There are three types of supplications that Allah will answer without a doubt. They are the supplication of one who has been wronged, even if the person who has been wronged is a disbeliever and was oppressed. If he supplicates to Allah, Allah will answer his supplication. The second is the supplication of a traveler, if he supplicates to Allah, the Exalted, during his journey. The third one is the supplication of a parent – a father or mother – whether for or against their child.