(a) It is narrated from Ibn al-Musayyib that the Messenger of Allah (peace and blessings be upon him) ordered Bilal (may Allah be pleased with him) to give the adhan; he put his fingers in his ears while the Messenger of Allah (peace and blessings be upon him) was watching, and he (peace and blessings be upon him) did not object to it. From that day this became the established Sunnah.
(b) It is narrated from Ibn Sirin that Bilal (may Allah be pleased with him) put his fingers in his ears during his adhan or iqamah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1857