Sahih Muslim, Chapter: Faith (Kitab Al Iman)

Verily the Messenger of Allah (may peace be upon him) said: Allah will admit into Paradise those deserving of Paradise, and He will admit whom He wishes out of His Mercy, and admit those condemned to Hell into the Fire (of Hell). He would then say: See, he whom you find having as much faith in his heart as a grain of mustard, bring him out. They will then be brought out burned and turned to charcoal, and would be cast into the river of life, and they would sprout aj does a seed in the silt carried away by flood. Have you not seen that it comes out yellow (fresh) and intertwined?