Musnad Ahmad 32:1455

Yunus narrated to us, Hammad—meaning Ibn Salamah—narrated to us, from Thabit, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) used to raid at the time of the dawn prayer, and he would listen for the Adhan. If he heard the 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." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Upon the Fitrah (natural disposition)." Then the man said: "I bear witness that there is no god but Allah." He (the Prophet) said: "He has exited from the Fire."