Musnad Ahmad 1153:1682

Waki’ narrated to us, he said: Sufyan and Isra’il narrated to us, from Abu Ishaq, from ‘Amr ibn Ghalib, who said: ‘Ammar came, and with him was al-Ashtar, seeking permission to enter upon ‘Aisha. He said: "O Mother." She said: "I am not a mother to you." He said: "Yes, even if you dislike it." She said: "Who is this with you?" He said: "This is al-Ashtar." She said: "Are you the one who intended to kill my sister’s son?" He said: "I intended to kill him, and he intended to kill me." She said: "As for if you had killed him, you would never have succeeded. I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'The blood of a Muslim man is not lawful except for one of three: a man who killed, so he is killed; or a man who committed adultery after having been married; or a man who apostatized after his Islam.'"