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He reported God’s Messenger as stating that if anyone says, “I am free from Islam,” that is just as he has said if he is lying, but if he is speaking the truth he will not return safely to Islam. Abu Dawud, Nasa’i and Ibn Majah transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " مَنْ قَالَ: إِنِّي بَرِيءٌ مِنَ الْإِسْلَامِ فَإِنْ كَانَ كَاذِبًا فَهُوَ كَمَا قَالَ وَإِنْ كَانَ صَادِقًا فَلَنْ يَرْجِعَ إِلَى الْإِسْلَامِ سَالِمًا ". رَوَاهُ أَبُو دَاوُدَ وَالنَّسَائِيُّ وَابْنُ مَاجَهْ
sunnah.com