(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
Musa bin Abi Uthman (Shu'bah said: he used to give the adhan on the tallest minaret in Kufah) said: Abu Yahya narrated to me while I was going round with him (that is, performing tawaf of the House of Allah); he said: I heard Abu Hurayrah (may Allah be pleased with him) say: I heard the Messenger of Allah (peace and blessings be upon him), I heard it from his (peace and blessings be upon him) own mouth: "The mu'adhdhin is forgiven to the extent of the reach of his voice, and every moist and dry thing testifies for him; and for the one who attends the prayer twenty-five good deeds are written, and (the sins) between them (the prayers) are expiated."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1861
It is narrated from Sayyiduna Abu Mahdhurah (may Allah be pleased with him): When Sayyiduna Umar (may Allah be pleased with him) came to Makkah, I gave the adhan, and Umar (may Allah be pleased with him) said to me: O Abu Mahdhurah, did you not fear that your belly (the part below the navel) would burst from straining?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1862