'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."

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ سَعْدِ بْنِ عَمَّارِ بْنِ سَعْدٍ، مُؤَذِّنِ رَسُولِ اللَّهِ ﷺ حَدَّثَنِي أَبِي عَنْ أَبِيهِ عَنْ جَدِّهِ أَنَّ رَسُولَ اللَّهِ ﷺ أَمَرَ بِلاَلاً أَنْ يَجْعَلَ إِصْبَعَيْهِ فِي أُذُنَيْهِ وَقَالَ " إِنَّهُ أَرْفَعُ لِصَوْتِكَ " .