Musannaf Ibn Abi Shayba 28:211

Abu Usama narrated to us, from Abd al-Hamid ibn Ja'far, from Yazid ibn Abi Habib, from Abu 'Alatha, that Umar ibn al-Khattab sent a military detachment, and they found a man from the Muslims who had become a Christian after his Islam, so they killed him. Umar was informed of that, so he said, "Did you invite him to Islam?" They said, "No." He said, "Then I am innocent before Allah of his blood."