Musnad Abu Ya'la al-Mawsili 115:114

Narrated to us Muhammad ibn al-Minhal al-Darir, narrated to us Yazid, narrated to us Sa'id and Hisham, from Qatadah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "He shall exit the Fire who said 'There is no god but Allah,' and there was in his heart of goodness the weight of a barleycorn. Then he shall exit the Fire who said 'There is no god but Allah,' and there was in his heart of goodness the weight of a wheat grain. Then he shall exit the Fire who said 'There is no god but Allah,' and there was in his heart of goodness the weight of an atom."