Sahih Bukhari Volume 001, Book 011, Hadith Number 583.

Narated By 'Abdul Rahman : Abu Sa'id Al-Khudri told my father, "I see you liking sheep and the wilderness. So whenever you are with your sheep or in the wilderness and you want to pronounce Adhan for the prayer raise your voice in doing so, for whoever hears the Adhan, whether a human being, a jinn or any other creature, will be a witness for you on the Day of Resurrection." Abu Said added, "I heard it (this narration) from Allah's Apostle."