Sahih Muslim Book 32, Hadith Number 6285.

Chapter : He upon whom Allah’s Apostle (may peace be upon him) invoked curse whereas he in fact did not deserve it, it would be a source of reward and mercy for him.

‘Aisha reported that two persons visited Allah’s Messenger (may peace be upon him) and both of them talked about a thing, of which I am not aware, but that annoyed him and he invoked curse upon both of them and hurled malediction, and when they went out I said: Allah’s Messenger, the good would reach everyone but it would not reach these two. He said: Why so? I said: Because you have invoked curse and hurled malediction upon both of them. He said: Don’t you know that I have made condition with my Lord saying thus: O Allah, I am a human being and that for a Muslim upon whom I invoke curse or hurl malediction make it a source of purity and reward.