Mishkat al-Masabih 4 Prayer كتاب الصلاة 92

وَعَنْ أَنَسٍ قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُغِيرُ إِذَا طَلَعَ الْفَجْرُ وَكَانَ يَسْتَمِعُ الْأَذَانَ فَإِنْ سَمِعَ أَذَانًا أَمْسَكَ وَإِلَّا أَغَارَ فَسَمِعَ رَجُلًا يَقُولُ اللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «عَلَى الْفِطْرَةِ» ثُمَّ قَالَ أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهِ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «خَرَجْتَ من النَّار» فنظروا فَإِذا هُوَ راعي معزى. رَوَاهُ مُسلم

Anas said that the Prophet used to attack the enemy at dawn, and he would sometimes hear the adhan , so if he heard an adhan he stopped, but otherwise he attacked. Once on hearing a man say, “God is most great, God is most great,” God’s Messenger said, “You follow Islam.” Then hearing him say, “There is no god but God,” he said, “You have come forth from hell.” They looked at him and found that he was a goatherd. Muslim transmitted it.