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]‏ .‏