Buraydah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: “Whoever swears saying: 'I have nothing to do with Islam,' if he is lying, then he will be as he said: and if he is telling the truth, then his Islam will be defective."
Hadīth explanation: Whoever swears saying that he has nothing to do with Islam, or saying that he is a Jew, a Christian, a disbeliever, or an atheist; then his case is either of the two: The first case: that he is lying in what he swears about; such as saying: “If such and such matter is not true, then I have nothing to do with Islam,” while he is lying about it. For example, saying: Zayd arrived today from his trip, and if it is not true then I have nothing to do with Islam, or I am a Jew, a Christian or a polytheist; while he knows that is lying (that Zayd did not arrive today). The second case: that he is telling the truth; for example, if he said: Zayd arrived today from his trip, and if it is not true then I have nothing to do with Islam, or I am a Jew or a Christian. If he is truthful (Zayd did arrive today), then, in this case, his Islam will not be perfect anymore as the Messenger (may Allah's peace and blessings be upon him) said. Rather, his Islam will be deficient because of those heinous words he uttered.