Musnad Ahmad 1238:0

Narrated to us by Waki’, from Shu’bah, from ‘Aasim ibn Kulayb, from his father, from Wa’il ibn Hujr, who said: I prayed with the Messenger of Allah (peace be upon him), and when he said "As-salamu ‘alaykum wa rahmatullah" to his right, he would say: "As-salamu ‘alaykum wa rahmatullah," and to his left: "As-salamu ‘alaykum wa rahmatullah." I saw him place his right hand upon his left hand in prayer.