Musnad Ahmad 32:1708

'Affan narrated to us, Hammad narrated to us, he said: Thabit informed us, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) used to raid at the time of the dawn prayer. He would listen; if he heard the call to prayer (Adhan), he would refrain, and if not, he would raid. One day he listened and heard a man saying: "Allah is the Greatest, Allah is the Greatest." He said: "The natural state (Fitrah)." Then he said: "I bear witness that there is no god but Allah." He said: "You have exited from the Fire."