Musnad Ahmad 32:1908

Affan narrated to us, Hammad narrated to us, from Thabit, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) heard a man saying: "Allah is the Greatest, Allah is the Greatest." He said: "Upon the natural disposition (fitrah)." Then he said: "I bear witness that there is no god but Allah, I bear witness that there is no god but Allah." He said: "You have exited from the Fire."