Bulugh al-Maram 2 The Book of Prayer كتاب الصلاة 39

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”.

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)].