Musnad Abu Ya'la al-Mawsili 159:102

Muhammad ibn Sahl ibn ‘Askar narrated to us, ‘Abdullah ibn Salih narrated to us, he said: Mu‘awiyah ibn Salih narrated to me, from Abu Maryam, from Abu Hurayrah, that he heard him saying: Whoever sees his brother, let him greet him with peace. If a tree, a wall, or a rock comes between them, and then he meets him, let him greet him with peace.