Sunan an-Nasa'i, Volume 1, Book 7, Hadith 644 (DarusSalam) ⓘ
It was narrated from 'Awn bin Abi Juhaifah that his father said: "I came to the Prophet and Bilal came out and called the Adhan and he started doing like this in his Adhan, turning to his right and left."
أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، قَالَ حَدَّثَنَا وَكِيعٌ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَوْنِ بْنِ أَبِي جُحَيْفَةَ، عَنْ أَبِيهِ، قَالَ أَتَيْتُ النَّبِيَّ صلى الله عليه وسلم فَخَرَجَ بِلاَلٌ فَأَذَّنَ فَجَعَلَ يَقُولُ فِي أَذَانِهِ هَكَذَا يَنْحَرِفُ يَمِينًا وَشِمَالاً .
also referenced under: Book 7, Hadith 650 (In-Book)