Anas (may Allah be pleased with him) says that the Prophet Muhammad (peace and blessings be upon him) used to attack at the time of Fajr prayer. If he heard the call to prayer, he would stop, otherwise he would attack. One day he listened carefully and heard a man saying: «الله أكبر الله أكبر» . The Prophet said: "He is upon the natural religion (Islam)." When the man said «أشهد أن لا إله إلا الله» , the Prophet said: "You have been saved from the Hellfire." Hasan says: Abu Al-Walid narrated to us, he said: Hammad bin Salamah narrated to us with this same chain and in a similar manner. Imam Tirmidhi says:
This hadith is Hasan Sahih.