Sunnah.com reference : Book 2, Hadith 39Arabic/English book reference : Book 2, Hadith 182
Narrated Abu Juhaifa (RA):
I saw Bilal calling for prayer and I would follow (looking at) his mouth (as he turned it) this (right) side and that (left) side with his fingers in his ears.

[Reported by Ahmad and at-Tirmidhi who graded it Sahih (authentic)].
وَعَنْ أَبِي جُحَيْفَةَ ‏- رضى الله عنه ‏- قَالَ: { رَأَيْتُ بِلَالاً يُؤَذِّنُ وَأَتَتَبَّعُ فَاهُ, هَاهُنَا وَهَاهُنَا, وَإِصْبَعَاهُ فِي أُذُنَيْهِ }  رَوَاهُ أَحْمَدُ, وَاَلتِّرْمِذِيُّ وَصَحَّحَهُ  1‏ .‏‏1 ‏- صحيح.‏ رواه أحمد (4/308‏-309)‏، والترمذي (197)‏، وقال الترمذي: "حديث حسن صحيح".‏