(3) باب السُّنَّةِ فِي الأَذَانِ
'Abdur-Rahman bin Sa'd bin 'Ammar bin Sa'd, who was the Mu'adh-dhin of the Messenger of Allah narrated from his grandfather, that:
The Messenger of Allah commanded Bilal to put his fingers in his ears when calling the Adhan, and he said: "It makes the voice louder."
Reference: Sunan Ibn Majah 710
In-book reference: Book 3, Hadith 5
English translation: Vol. 1, Book 3, Hadith 710