Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) was on one of his journeys when he (peace and blessings be upon him) heard a caller saying: Allahu akbar, Allahu akbar. So the Messenger of Allah (peace and blessings be upon him) said: "He is upon the fitrah (natural disposition)." Then he said: I bear witness that there is no god but Allah. So the Messenger of Allah (peace and blessings be upon him) said: "He has come out of Hellfire."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10027