Musnad Ahmad 1226:5

Safwan narrated to us, he said: Abdullah bin Sa'id informed us, from his father, from Abu Shurayh bin 'Amr al-Khuza'i, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware of sitting on the pathways. So whoever among you sits on the pathway, let him give it its right." He said: We said: "O Messenger of Allah, and what is its right?" He said: "Lowering the gaze, returning the greeting, enjoining good, and forbidding evil."