From him, from Muhammad ibn Al-Husayn, from Ja'far ibn Bashir, from Al-Hasan ibn Al-Sarri:
Abu Abdillah عليه السلام, peace be upon him, said: "The Sunnah is to place your two fingers in your ears during the Adhan."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 307