Sahih Muslim Book - 4, Hadith - 847

و حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ حَدَّثَنَا يَحْيَى يَعْنِي ابْنَ سَعِيدٍ عَنْ حَمَّادِ بْنِ سَلَمَةَ حَدَّثَنَا ثَابِتٌ عَنْ أَنَسِ بْنِ مَالِكٍ قَالَ كَانَ رَسُولُ اللَّهِ 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 اللَّهُ أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ خَرَجْتَ مِنْ النَّارِ فَنَظَرُوا فَإِذَا هُوَ رَاعِي مِعْزًى.

It was narrated that Anas bin Malik said: "The Messenger of Allah (s.a.w) used to attack at dawn (during military expeditions), so that he could listen out for the Adhan. If he heard the Adhan then he would refrain from attacking, otherwise he would attack. He heard a man saying: 'Allahu akbaru Allahu akbar' and the Messenger of Allah (s.a.w) said: 'He is following the Fitrah.' Then he said: 'Ashhadu an la ilaha illallah, Ashhadu an la ilaha illallah (I bear witness that none has the right to be worshiped but Allah, I bear witness that none has the right to be worshiped but Allah).' The Messenger of Allah (s.a.w) said: 'You have escaped the Fire.' They looked, and saw that he was a goatherd."