Musnad Abu Ya'la al-Mawsili 160:231

Ubayd Allah ibn Umar al-Jushami narrated to us, Yazid ibn Zuray' narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from Sa'id al-Maqburi, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade the courtyards and the pathways from being sat in. The Muslims said to him, "We cannot avoid that." He said, "If you must, then give them their right." They said, "And what is their right?" He said, "Returning the greeting, saying 'Yarhamuk Allah' (may Allah have mercy on you) to the one who sneezes if he praises Allah the Almighty, lowering the gaze, and guiding the way."