(48) باب مَا جَاءَ فِي وَصِيَّتِهِ صلى الله عليه وسلم فِي الْقِتَالِ
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."
Reference: Jami` at-Tirmidhi 1618
In-book reference: Book 21, Hadith 82
English translation: Vol. 3, Book 19, Hadith 1618