سنن ابن ماجه

'Abdur-Rahman bin Sa'd bin 'Ammar bin Sa'd, who was the Mu'adh-dhin of the Messenger of Allah (ﷺ) narrated from his grandfather, that: The Messenger of Allah (ﷺ) commanded Bilal to put his fingers in his ears when calling the Adhan, and he said: "It makes the voice louder."