Musannaf Ibn Abi Shayba 17:1248

Ibn Numayr narrated to us, he said: Musa bin Ubayd Allah narrated to us, from Ayyub bin Khalid, from Malik bin al-Tayyihan, who said: A group of us gathered at the Messenger of Allah (may Allah bless him and grant him peace), and we said: "O Messenger of Allah, we are people of the lowlands and people of the highlands; we sit in these gatherings, so what do you command us?" He said: "Give the gatherings their right." He said: "Lower your gazes, return the greeting, guide the blind, enjoin the good, and forbid the evil."