It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that once we were on a journey with the Prophet (peace and blessings be upon him) when we heard a caller saying: «الله اكبر الله اكبر» , the Prophet (peace and blessings be upon him) replied to this by saying: "This person is upon sound nature (fitrah)." Then he said: «أشهد أن لا إلٰه إلا الله» , so the Prophet (peace and blessings be upon him) said, "He has been saved from the Fire." We hurried towards him and found that he was a shepherd who had caught the prayer and it was he who had made this call.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 3861