Musnad Ahmad 30:1674

Khalaf narrated to us, he said: Abu Ma'shar narrated to us, from Sa'id al-Maqburi, from Abu Hurayrah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The supplication of the oppressed is answered, even if he is a wicked person, for his wickedness is upon himself."