Musannaf Ibn Abi Shayba 3:671

Ibn Idris narrated to us, from al-Hasan ibn ‘Amr, who said: The taslim (salutation) was mentioned in the presence of Shaqiq, and he said: I have prayed behind ‘Umar and ‘Abdullah, and both of them used to say: "As-salamu ‘alaykum wa rahmatullah, as-salamu ‘alaykum wa rahmatullah."