Narrated Anas (Radhiallaho anho):
The Prophet (sallallaahu 'alaihi wasallam) said, "Whoever said
"None has the right to be worshipped but Allah and has in his
heart good (faith) equal to the weight of a barley grain will
be taken out of Hell. And whoever said: "None has the right to
be worshipped but Allah and has in his heart good (faith)
equal to the weight of a wheat grain will be taken out of Hell.
And whoever said, "None has the right to be worshipped but
Allah and has in his heart good (faith) equal to the weight of
an atom will be taken out of Hell."

Bukhari Vol. 1 : No. 42