An example of the supplication of the oppressed being accepted is the incident of Saad (RA) with Abu Sa'dah. When asked about Saad (RA), Sa’dah said, "As you have put us under an oath; I am bound to tell you that Sa`d never went himself with the army and never distributed (the war booty) equally and never did justice in legal verdicts." (On hearing it) Sa`d said, "I pray to Allah for three things: O Allah! If this slave of yours is a liar and got up for showing off, give him a long life, increase his poverty and put him to trials." (And so it happened). Later on when that person was asked how he was, he used to reply that he was an old man in trial as the result of Sa`d's curse. `Abdul Malik (RA), the sub narrator, said that he had seen him afterwards and his eyebrows were overhanging his eyes owing to old age and he used to tease and assault the small girls in the way.

Reference: Bukhari: 755