Musnad Abu Ya'la al-Mawsili 115:86

Ubayd Allah narrated to us, Mu'adh ibn Hisham and Abu 'Amir al-'Aqdi both narrated to us, they said: Hisham narrated to us, from Qatadah, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "He who says 'There is no god but Allah' and has in his heart of goodness the weight of a grain of wheat shall be brought out of the Fire. Then, he who says 'There is no god but Allah' and has in his heart the weight of an atom shall be brought out of the Fire."