(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
It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to Sayyiduna Bilal (may Allah be pleased with him): "O Bilal, when you give the adhan, give it slowly, and when you give the iqamah, say it quickly. Leave between your adhan and your iqamah as much time as it takes one who is eating to finish his food, one who is drinking to finish his drink, and one who goes to relieve himself to finish relieving himself. And do not stand up until you see me." (b) Imam al-Bukhari (may Allah have mercy on him) said: Abd al-Mun'im is munkar al-hadith (his narrations are denounced), and Yahya bin Muslim al-Bakka' al-Kufi was declared da'if (weak) by Yahya bin Ma'in.
It has also been narrated with another chain from al-Hasan and Ata from Sayyiduna Abu Hurayrah (may Allah be pleased with him), but it is not well known.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2008
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to Sayyiduna Bilal (may Allah be pleased with him) the like of the previous hadith, which he mentioned up to the words "to relieve himself". And the first chain is better known than this one.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2009
Abu al-Zubayr, who was the muezzin of Bayt al-Maqdis, said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said to me: When you give the adhan, give it slowly, and when you give the iqamah, say it quickly.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2010