Sayyiduna Anas (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) used to launch an attack at the time of the morning prayer. He (peace be upon him) would first listen attentively; if he (peace be upon him) heard the sound of the adhan, he (peace be upon him) would stop, otherwise he would attack. The narrator states: Once, he (peace be upon him) tried to listen and a man was saying "Allahu Akbar, Allahu Akbar," so the Prophet (peace be upon him) said: "He is upon the natural disposition (i.e., upon the religion of fitrah)." That person said: "I bear witness that there is no deity except Allah," so the Prophet (peace be upon him) said: "He has been saved from the Hellfire."
Hadith Referenceصحیح ابن حبان / كتاب السير / 4753
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (2368): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم