It was narrated from 'Awn bin Abi Juhaifah that his father said:
"I came to the Prophet (ﷺ) and Bilal came out and called the Adhan and he started doing like this in his Adhan, turning to his right and left."
English reference: Vol. 1, Book 7, Hadith 644
Arabic reference: Book 7, Hadith 650