Sayyidna Abu Hurayra, Radi-Allahu anhu, narrates that Rasulullah said, 'Three Du'as are accepted without a shadow of doubt:
a) The Du'a of the parents,
b) The Du'a of the travellers,
c) Du'a of the oppressed. (Mishkaat pg.195; Delhi)

by Abu Abdullah Ibne Ismail
“One should abstain from all types of oppression. This is because the dua of the oppressed is always accepted. It is important to realise that this does not mean that an oppressed person has to make dua verbally.
Just a sigh of hurt and pain from the oppressed one is enough for the wrath of Allah ta’ala to descend on those who are oppressors.”