Sahih Muslim , Book 4, Hadith 745 (USC-MSA) ⓘ

Anas b. Malik reported: The Messenger of Allah used to attack the enemy when it was dawn. He would listen to the Adhan; so if he heard an Adhan, he stopped, otherwise made an attack. Once on hearing a man say: Allah is the Greatest, Allah is the Greatest, the Messenger of Allah remarked: He is following al-Fitra (al-Islam). Then hearing him say: I testify that there is no god but Allah. there is no god but Allah, the Messenger of Allah said: You have come out of the Fire (of Hell). They looked at him and found that he was a goat herd.

وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا يَحْيَى، - يَعْنِي ابْنَ سَعِيدٍ - عَنْ حَمَّادِ بْنِ سَلَمَةَ، حَدَّثَنَا ثَابِتٌ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُغِيرُ إِذَا طَلَعَ الْفَجْرُ وَكَانَ يَسْتَمِعُ الأَذَانَ فَإِنْ سَمِعَ أَذَانًا أَمْسَكَ وَإِلاَّ أَغَارَ فَسَمِعَ رَجُلاً يَقُولُ اللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ عَلَى الْفِطْرَةِ ‏"‏ ‏.‏ ثُمَّ قَالَ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ خَرَجْتَ مِنَ النَّارِ ‏"‏ ‏.‏ فَنَظَرُوا فَإِذَا هُوَ رَاعِي مِعْزًى ‏.‏