Maliks Muwatta Book 36, Hadith Number 16.
Section : Judgement on Abandonment of Islam.
Malik related to me from Abd ar-Rahman ibn Muhammad ibn Abdullah ibn 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, and he informed him. Then Umar inquired, ‘Do you have any recent news?’ He said, ‘Yes. A man has become a kafir after his Islam.’ Umar asked, ‘What have you done with him?’ He said, ‘We let him approach and struck off his head.’ Umar said, ‘Didn’t you imprison him for three days and feed him a loaf of bread every day and call on him to tawba that he might turn in tawba and return to the command of Allah?’ Then Umar said, ‘O Allah! I was not present and I did not order it and I am not pleased since it has come to me!'”