Malik related to me from Abd ar-Rahman ibn Muhammad ibn Abdullahibn Abd al-Qari that his father said, "A man came to Umar ibn al-Khattab from Abu Musa al-Ashari. Umar asked after various people, andhe informed him. Then Umar inquired, 'Do you have any recent news?' Hesaid, 'Yes. A man has become a kafir after his Islam.' Umar asked,'What have you done with him?' He said, 'We let him approach andstruck off his head.' Umar said, 'Didn't you imprison him for threedays and feed him a loaf of bread every day and call on him to tawbathat he might turn in tawba and return to the command of Allah?' ThenUmar said, 'O Allah! I was not present and I did not order it and I amnot pleased since it has come to me!' "
USC-MSA web (English) reference: Book 36, Hadith 16
Arabic reference: Book 36, Hadith 1420