Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 165
It is narrated on the authority of 'Abdullah b. Mas'ud that the Messenger of Allah (may peace be upon him) observed: None shall enter the Fire (of Hell) who has in his heart the weight of a mustard seed of Iman and none shall enter Paradise who has in his heart the weight of a mustard seed of pride.