Sayyiduna Bilal (may Allah be pleased with him) states that the Prophet (peace and blessings be upon him) said: "When you give the adhan, put your fingers in your ears, for that will make your voice louder."
Imam al-Tabarani recorded this narration in al-Mu'jam al-Kabir. Its chain contains a narrator, Abd al-Rahman bin Ammar, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1887