Sahih Bukhari Volume 9, Book 93, Hadith Number 638.

Narrated By ‘Abdullah bin ‘Abdur-Rahman : That Abu Sa’id Al-Khudri said to him, “I see that you like sheep and the desert, so when you are looking after your sheep or when you are in the desert and want to pronounce the Adhan, raise your voice, for no Jinn, human being or any other things hear the Mu’adh-dhin’s voice but will be a witness for him on the Day of Resurrection.” Abu Sa’id added, “I heard this from Allah’s Apostle.”