The Prophet said, "Whoever said "None has the right to be worshiped 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 worshiped 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 worshiped but Allah and has in his heart good (faith) equal to the weight of an atom will be taken out of Hell."
Source: Sahih Bukhari