Musnad Ahmad 1101:4

Isma'il ibn Ibrahim narrated to us, from 'Abd al-Rahman ibn Ishaq, from al-Zuhri, from 'Ata' ibn Yazid, from 'Ubayd Allah ibn 'Adi ibn al-Khiyar, from al-Miqdad ibn 'Amr, who said: I said: "O Messenger of Allah, have you considered a man who strikes me with a sword, cutting off my hand, then he takes refuge from me behind a tree and says: 'There is no god but Allah'—should I kill him?" He said: "No." I repeated it two or three times, and he said: "No, unless he is like you before he said what he said, and you are like him before you did what you did."