Musnad Ahmad 1153:295

Ibn Numayr narrated to us, Yunus bin Abi Ishaq narrated to us, from Abi Ishaq, from ‘Amr bin Ghalib, who said: I arrived at ‘Aishah’s [residence] along with ‘Ammar and al-Ashtar. ‘Ammar said: "Peace be upon you, O mother." She replied: "Peace be upon those who follow guidance," and he repeated it to her two or three times. Then he said: "By Allah, you are indeed my mother, 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 wanted to kill my sister’s son?" He said: "Yes, I wanted that, and he wanted it." She said: "Had you done so, you would not have succeeded." As for you, O ‘Ammar, I heard—or [I heard] the Messenger of Allah, may Allah bless him and grant him peace, say: "The blood of a Muslim man is not lawful except for one of three [reasons]: except for one who commits adultery after having been chaste, or one who disbelieves after having accepted Islam, or one who kills a soul, so he is killed for it."