Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(19)
Chapter: Should the Mu'adh-dhin turn his mouth (face) and look from side to side during the Adhan?
(19)
باب هَلْ يَتَتَبَّعُ الْمُؤَذِّنُ فَاهُ هَا هُنَا وَهَا هُنَا، وَهَلْ يَلْتَفِتُ فِي الأَذَانِ
Sahih al-Bukhari 634

Narrated `Aun bin Abi Juhaifa:

My father said, "I saw Bilal turning his face from side to side while pronouncing the Adhan for the prayer."

حَدَّثَنَا مُحَمَّدُ بْنُ يُوسُفَ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَوْنِ بْنِ أَبِي جُحَيْفَةَ، عَنْ أَبِيهِ، أَنَّهُ رَأَى بِلاَلاً يُؤَذِّنُ فَجَعَلْتُ أَتَتَبَّعُ فَاهُ هَا هُنَا وَهَا هُنَا بِالأَذَانِ‏.‏
Reference : Sahih al-Bukhari 634
In-book reference : Book 10, Hadith 31
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 607
  (deprecated numbering scheme)