بُلُوغُ الْمَرَامِ
Bulugh al-Maram 182
The Book of Prayer
وَعَنْ أَبِي جُحَيْفَةَ - رضى الله عنه - قَالَ: { رَأَيْتُ بِلَالاً يُؤَذِّنُ وَأَتَتَبَّعُ فَاهُ, هَاهُنَا وَهَاهُنَا, وَإِصْبَعَاهُ فِي أُذُنَيْهِ } رَوَاهُ أَحْمَدُ, وَاَلتِّرْمِذِيُّ وَصَحَّحَهُ [1] .
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)].
Sunnah.com reference: Book 2, Hadith 39