The Book of Prayer
Bulugh al-Maram #218
كتاب الصلاة
وَعَنْ أَبِي جُحَيْفَةَ - رضى الله عنه - قَالَ: «رَأَيْتُ بِلَالاً يُؤَذِّنُ وَأَتَتَبَّعُ فَاهُ, هَاهُنَا وَهَاهُنَا, وَإِصْبَعَاهُ فِي أُذُنَيْهِ» رَوَاهُ أَحْمَدُ, وَاَلتِّرْمِذِيُّ وَصَحَّحَهُ
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.
وَعَنْ أَبِي جُحَيْفَةَ - رضى الله عنه - قَالَ: "رَأَيْتُ بِلَالاً يُؤَذِّنُ وَأَتَتَبَّعُ فَاهُ, هَاهُنَا وَهَاهُنَا, وَإِصْبَعَاهُ فِي أُذُنَيْهِ" رَوَاهُ أَحْمَدُ, وَاَلتِّرْمِذِيُّ وَصَحَّحَهُ
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.
— Bulugh al-Maram, Hadith 218