Narrated Anas bin Malik:
That the Prophet (ﷺ) would not attack except near the time of Fajr, so if he heard the Adhan he would refrain, and if not, then he would attack. So he listened one day and heard a man saying: Allahu Akbar, Allahu Akbar, so he said: Upon the Fitrah. Then he said: I bear witness that none has the right to be worshipped but Allah. So he said: You have departed from the Fire.
Another chain with similar narrations.
[Abu 'Eisa said:
] This Hadith is Hasan Sahih.