Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 213
It is narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) 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.