Musnad Ahmad 32:1588

Mu'ammal narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas, who said: While we were with the Messenger of Allah, may Allah bless him and grant him peace, on a journey, he heard a man saying: "Allah is the Greatest, Allah is the Greatest." The Prophet, may Allah bless him and grant him peace, said: "Upon the natural disposition (fitrah)." Then the man said: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah." The Prophet, may Allah bless him and grant him peace, said: "This one has exited from the Fire."