Musnad Ahmad Hadith 5708

Abdur Rahman bin Samurah (may Allah have mercy on him) says that once I was walking with Ibn Umar (may Allah be pleased with them), and on the way, they passed by a severed head that was hanging on a gallows. Upon seeing it, he said, "The one who killed him is wretched." I said to him, "O Abu Abdur Rahman! Are you saying this?" As soon as he heard this, he withdrew his hand from mine and said, "I heard the Prophet (peace and blessings of Allah be upon him) say: 'If any of my followers sets out to kill someone and kills him in such a manner (as the head of this murdered person is hanging), then the murdered person will go to Paradise and the killer to Hell.'"