Musnad Ahmad 32:829

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, Qatadah narrated to us, and Hajjaj said: Shu'bah narrated to me, from Qatadah, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Bring out of the Fire whoever said: 'There is no god but Allah,' who has in his heart of goodness the weight of a barleycorn. Bring out of the Fire whoever said: 'There is no god but Allah,' who has in his heart of goodness the weight of an atom. Bring out of the Fire whoever said: 'There is no god but Allah,' who has in his heart of goodness the weight of a wheat grain."