It was narrated from 'Abdullah that: The Prophet (ﷺ) used to say salam to his right and to his left: As-salamu 'alaykum wa rahmatullah, as-salamu alaykum wa rahmatullah (peace be upon you and the mercy of Allah, peace be upon you and the mercy of Allah) until the whiteness of his cheek could be seen from here, and the whiteness of his cheek from here.
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، عَنْ سُفْيَانَ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الأَحْوَصِ، عَنْ عَبْدِ اللَّهِ، عَنِ النَّبِيِّ صلى الله عليه وسلم . أَنَّهُ كَانَ يُسَلِّمُ عَنْ يَمِينِهِ وَعَنْ يَسَارِهِ " السَّلاَمُ عَلَيْكُمْ وَرَحْمَةُ اللَّهِ السَّلاَمُ عَلَيْكُمْ وَرَحْمَةُ اللَّهِ " . حَتَّى يُرَى بَيَاضُ خَدِّهِ مِنْ هَا هُنَا وَبَيَاضُ خَدِّهِ مِنْ هَا هُنَا .
sunnah.com