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 " .
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."
Source: Jami` at-Tirmidhi — repository translation · Imam Abu Isa Muhammad ibn Isa al-Tirmidhi — authenticity/grade metadata requires independent source review · Sunnah.comAudio not available for the selected reciter.
Source: Jami` at-Tirmidhi — repository arabic · Imam Abu Isa Muhammad ibn Isa al-Tirmidhi · Sunnah.com