Musnad Ahmad 32:607

Yahya ibn Ishaq narrated to us, he said: Yahya ibn Ayyub informed me, he said: Abu Abdullah al-Asadi informed me, he said: I heard Anas ibn Malik saying: The Messenger of Allah, may Allah bless him and grant him peace, said: "Beware of the supplication of the oppressed, even if he is a disbeliever, for there is no veil between it and Allah."