Musnad Abu Ya'la al-Mawsili 117:33

Ibrahim ibn al-Hajjaj narrated to us, Hammad narrated to us, from Thabit, from Anas: That the Messenger of Allah (ﷺ) used to raid at the time of the morning prayer, and he would listen for the Adhan. If he heard an 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 (ﷺ) said, "Upon the Fitrah (natural disposition)." The man said, "I bear witness that there is no god but Allah." The Messenger of Allah (ﷺ) said, "You have exited the Fire."