´It was narrated from 'Awn bin Abi Juhaifah that his father said:` "I came to the Prophet (S.A.W) and Bilal came out and called the Adhan and he started doing like this in his Adhan, turning to his right and left."
Explanation & Benefits
Hafiz Muhammad Ameen
644. Commentary: Although the call to prayer (adhan) is generally given facing the qiblah, at the time of saying «حيَّ على الصلاةِ» the face is turned to the right, and at the time of saying «حيَّ على الفلاحِ» the face is turned to the left, so that the voice may reach both the right and left sides as well. This is a Sunnah. Some people think that this was a temporary necessity which has now been fulfilled with the invention of the loudspeaker, and therefore, there is no longer any need to turn to the right or left. However, this explanation is completely contrary to the Prophetic method. Whether or not there is any apparent wisdom in it, in any case, there is goodness and benefit only in adhering to the ways of the Prophet sallallahu alayhi wa sallam.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 644