Musnad Abu Ya'la al-Mawsili 115:48

Abu Bakr ibn Abi Shaybah narrated to us, Muhammad ibn Bishr narrated to us, from Sa'id, from Qatadah, from Anas, that the Prophet, peace and blessings of Allah be upon him, 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 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 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."