(a) Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) used to say: I see that you love sheep and the open country. So when you are among your sheep or in the open country, give the adhan for the prayer and raise your voice in the call, for "No jinn or human hears the extent of the mu'adhdhin's voice except that he will testify for him on the Day of Resurrection."
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) Abd al-Rahman bin Abdullah bin Abi Sa'sa'ah narrates from his father that Abu Sa'id al-Khudri (may Allah be pleased with him) said to him: Give the adhan for the prayer and raise your voice, for whoever hears the reach of a mu'adhdhin's voice (will testify for him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1860