Jami` at-Tirmidhi, Book 21, Hadith 82 (In-Book) ⓘ

Narrated Anas bin Malik: That the Prophet 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."

حَدَّثَنَا الْحَسَنُ بْنُ عَلِيٍّ الْخَلاَّلُ، حَدَّثَنَا عَفَّانُ، حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، حَدَّثَنَا ثَابِتٌ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم لاَ يُغِيرُ إِلاَّ عِنْدَ صَلاَةِ الْفَجْرِ فَإِنْ سَمِعَ أَذَانًا أَمْسَكَ وَإِلاَّ أَغَارَ فَاسْتَمَعَ ذَاتَ يَوْمٍ فَسَمِعَ رَجُلاً يَقُولُ اللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ ‏.‏ فَقَالَ ‏"‏ عَلَى الْفِطْرَةِ ‏"‏ ‏.‏ فَقَالَ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ ‏.‏ فَقَالَ ‏"‏ خَرَجْتَ مِنَ النَّارِ ‏"‏ ‏.‏