Musnad Ahmad 30:358

Yazid narrated to us, Ibn 'Awn informed us, from Muhammad, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The angels curse one of you if he points at his brother with a piece of iron, even if he is his brother through his father and mother." Ibn Abi 'Adi did not attribute it to the Prophet (i.e., he narrated it as a statement of the Companion).