The Chapters on Expiation
11/3. Chapter:
One who takes an oath to follow a religion other than Islam
Sunan Ibn Majah
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا بَقِيَّةُ، عَنْ عَبْدِ اللَّهِ بْنِ مُحَرَّرٍ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، قَالَ سَمِعَ النَّبِيُّ ـ صلى الله عليه وسلم ـ رَجُلاً يَقُولُ أَنَا إِذًا لَيَهُودِيٌّ . فَقَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " وَجَبَتْ " .
It was narrated from Anas that the Messenger of Allah (ﷺ) heard a man say: "If that happens, I will be a Jew." The Messenger of Allah (ﷺ) said: 'That is guaranteed."
Reference : Sunan Ibn Majah 2099In-book reference : Book 11, Hadith 10English translation : Vol. 3, Book 11, Hadith 2099