Musnad Ahmad 32:411

'Abd al-Rahman narrated to us, Hammad ibn Salamah narrated to us, from Thabit, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to launch raids 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. He said: So he listened one day, and he 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." He said: "You have exited the Fire."