Musannaf Ibn Abi Shayba 3:680

Ibn Fudayl narrated to us, from Yazid ibn Abi Ziyad, from Ibrahim, that he used to say the taslim to his right: "As-salamu alaykum wa rahmatullah," raising his voice with it. And to his left: "As-salamu alaykum wa rahmatullah," lower than the first.