Abd al-A'la (may Allah have mercy on him) said: I prayed behind Abd al-Rahman al-Yahsubi (may Allah have mercy on him), and he gave the salam to the right saying "As-salamu alaykum wa rahmatullah," and to the left in the same way. I asked him: Where did you take this from? He said: I prayed behind Sayyiduna Wa'il bin Hujr (may Allah be pleased with him), and he said: I prayed behind the Messenger of Allah (peace and blessings be upon him), and he did the same, until I saw the whiteness of both his cheeks.
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18326