If a person has been wronged by someone, Allah accepts the du’a of that person against the person who has wronged them. This ‘wrong’ could be in respect to one’s rights, oppression, persecution, slander or any other type of wrong that a person could be inflicted with.
The Prophet, peace and blessings of Allah be upon him, said:
“Three people’s du’as are never rejected: the one who is fasting, until he breaks his fast; the just ruler; and the one who has been wronged. Allah raises it above the clouds, and the doors of the skies are opened for it, and the Lord says, ‘By my Honour and Glory! I will help you, even if it be after some time!'” (Recorded in al-Tirmidhi)
This is a very severe warning to the one who wrongs others, and a comfort and solace to the one who has been wronged. All praises be to Allah Who has sworn that He will definitely come to the aid of the one who is wronged.
The Prophet, peace and blessings of Allah be upon him, mentioned in numerous ahadith that a Muslim should fear the du’a of the one who has been wronged:
“Fear the du’a of he who has been wronged, for verily it ascends to the skies faster than sparks (of light).” (Recorded in al-Hakim)
“Fear the du’a of he who has been wronged, even if he is a disbeliever, for there remains no veil between it.” (Recorded in Ahmad)
This hadith shows us that there is no difference between doing wrong to a Muslim or to a disbeliever, and that both their du’as are answered by Allah. If this is the case of the disbeliever, then how much more so the case of the sinful Muslim? We should never despair in the mercy of Allah.