Musnad Ahmad 1017:72

Zayd ibn al-Hubab narrated to us from his book, Husayn narrated to me, Ibn Buraydah narrated to me, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever swears that he is free from Islam, if he is a liar, then he is as he said. And if he is truthful, he will never return to Islam safely."