Musnad Ahmad 26:691

Waki’ and ‘Abd al-Rahman narrated to us, they said: Sufyan narrated to us, from Abu Ishaq, from Abu al-Ahwas, from ‘Abdullah, from the Prophet (peace and blessings of Allah be upon him), that: "He used to say the taslim to his right and to his left: 'As-salamu 'alaykum wa rahmatullah' (Peace be upon you and the mercy of Allah), 'As-salamu 'alaykum wa rahmatullah' (Peace be upon you and the mercy of Allah), until the whiteness of his cheek could be seen." And ‘Abd al-Rahman said: "Until the whiteness of his cheek could be seen from here, and the whiteness of his cheek could be seen from here."