Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 212
It is narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) said: Verily Allah would make a wind to blow from the side of the Yemen more delicate than silk and would spare none but cause him to die who, in the words of Abu 'Alqama, has faith equal to the weight of a grain; while Abdul-'Aziz said: having faith equal to the weight of a dust particle.