It is narrated from Sayyiduna Safwan bin Assal (may Allah be pleased with him), who said: We were with the Messenger of Allah (peace and blessings be upon him) when he (peace and blessings be upon him) heard a man giving the call to prayer, and the Prophet (peace and blessings be upon him) said: "He is upon the fitrah (natural disposition)." When he said: I bear witness that there is no true god except Allah, he (peace and blessings be upon him) said: "He has testified to the truth." When he said: I bear witness that Muhammad (peace and blessings be upon him) is the Messenger of Allah, he (peace and blessings be upon him) said: "He has come out of the Fire."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7361