Reference: Fataawa wa Rasaa.il Mukhtaarah – Pages 539-540
Question: Sometimes when I become angry with someone I supplicate against them that Allaah does not grant him success, or any other (such) evil supplication. So what is the ruling regarding that?
Response: It is not befitting for a Muslim to supplicate against his Muslim brother; That is because if the right was with him – that he had been oppressed or otherwise – then he has taken his right with his supplication. If, however, he maintains his right (without supplicating against his Muslim brother) then that is better, because this will benefit him in the Hereafter.
And Allaah [alone] has the Complete knowledge [of all affairs].