Mishkat al-Masabih #606

‘Abd ar-Rahman b Sa'd b. 'Ammar b. Sa‘d, the mu’adhdhin* of God’s Messenger, said that his father told him from his father from his grandfather that God’s Messenger commanded Bilal to put his fingers in his ears, saying that it made the voice louder. * The mu’adhdhin here mentioned was Sa‘d, great-grandfather of ‘Abd ar-Rahman. Ibn Majah transmitted it.