It is narrated from Sayyiduna Wahshi bin Harb (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Perhaps after me you will conquer great cities and set up gatherings in their marketplaces. When that happens, return the greeting of peace (salam), lower your gaze, guide the blind, and help the oppressed."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18587