(a) It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that he said: I see that you love sheep and the countryside. When you are among your sheep and in your countryside and give the adhan for prayer, raise your voice when calling, for "no jinn, human or anything else hears the farthest reach of your voice except that it will bear witness for you on the Day of Resurrection." Sayyiduna Abu Sa'id (may Allah be pleased with him) said: I heard this from the Messenger of Allah (peace and blessings be upon him).
(b) Imam al-Shafi'i (may Allah have mercy on him) said: The encouragement to raise the voice is evidence for pronouncing (the words of) the adhan slowly and deliberately.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2007