عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " بَادِرُوا بِالأَعْمَالِ فِتَنًا كَقِطَعِ اللَّيْلِ الْمُظْلِمِ يُصْبِحُ الرَّجُلُ مُؤْمِنًا وَيُمْسِي كَافِرًا أَوْ يُمْسِي مُؤْمِنًا وَيُصْبِحُ كَافِرًا يَبِيعُ دِينَهُ بِعَرَضٍ مِنَ الدُّنْيَا " .
It is narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) observed: Be prompt in doing good deeds (before you are overtaken) by turbulence which would be like a part of the dark night. During (that stormy period) a man would be a Muslim in the morning and an unbeliever in the evening or he would be a believer in the evening and an unbeliever in the morning, and would sell his faith for worldly goods.
(Sahih Muslim, hadith number: 118)
Explanation of this hadith can be heard on this youtube 5 min and 49 video:
"A Man Wakes Up A Believer and Goes To Bed A Kafir" - Shaykh Salih Al-Fawzan hafithahullah.