Musnad Ahmad 30:1441

'Affan narrated to us, Shu'bah narrated to us, Suhayl ibn Abi Salih informed me, he said: I went out with my father to Ash-Sham, and the people of Ash-Sham would pass by the people of the hermitages and greet them. So I heard my father saying: I heard Abu Hurayrah saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Do not initiate the greeting of peace (salam) to them, and force them to the narrowest part of the road."