Musnad Ahmad 25:834

Hassan narrated to us, Hammad ibn Salamah narrated to us, from Thabit al-Bunani, from Anas ibn Malik, from the Prophet, peace and blessings of Allah be upon him, the like of it. He said in the first: "Whoever has in his heart the weight of a barleycorn of faith." And the second: "A wheat grain." And the third: "An atom."