Musnad Ahmad 1017:76

Yahya ibn Wadih Abu Tamila narrated to us, Husayn ibn Waqid informed me, he said: I heard Ibn Buraydah saying: I heard my father saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever says: 'I am innocent of Islam,' then if he is a liar, he is as he said, and if he is truthful, he will never return to Islam."