Hadithcore
Sahih al-Bukhari, 7114
sahih

Narrated Abi Asha'sha:

Hudhaifa said, 'In fact, it was hypocrisy that existed in the lifetime of the Prophet (ﷺ) but today it is Kufr (disbelief) after belief.'

حَدَّثَنَا خَلاَّدٌ، حَدَّثَنَا مِسْعَرٌ، عَنْ حَبِيبِ بْنِ أَبِي ثَابِتٍ، عَنْ أَبِي الشَّعْثَاءِ، عَنْ حُذَيْفَةَ، قَالَ إِنَّمَا كَانَ النِّفَاقُ عَلَى عَهْدِ النَّبِيِّ صلى الله عليه وسلم فَأَمَّا الْيَوْمَ فَإِنَّمَا هُوَ الْكُفْرُ بَعْدَ الإِيمَانِ‏.‏

References2 variants
In-Book Reference
Book 92, Hadith 61
USC-MSA web (English) reference
Vol. 9, Book 88, Hadith 230 (deprecated numbering scheme)