Musannaf Ibn Abi Shayba 28:216

'Abd al-Rahim ibn Sulayman narrated to us, from Hajjaj, from 'Amr ibn Shu'ayb, from his father, from his grandfather, who said: 'Amr ibn al-'As wrote to 'Umar ibn al-Khattab that a man had exchanged faith for disbelief. 'Umar wrote back to him: "Seek his repentance; if he repents, accept it from him, and if not, strike his neck."