Musnad Ahmad 2:17

Aswad ibn Amir narrated to us, he said: Ja'far—meaning al-Ahmar—informed us, from Mutarrif, from al-Hakam, from Mujahid, who said: A man threw a sword at his son and killed him. He was brought before Umar, who said: "Were it not that I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'A father shall not be executed for [killing] his child,' I would have executed you before you departed."