Musnad Abu Ya'la al-Mawsili 160:109

And with his chain of narration, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The right of the Muslim upon the Muslim is six." It was said: "What are they, O Messenger of Allah?" He said: "If you meet him, then greet him; and if he invites you, then answer him; and if he seeks your advice, then advise him; and if he sneezes and praises Allah (the Mighty and Majestic), then say 'Yarhamuk Allah' (may Allah have mercy on you) to him; and if he falls ill, then visit him; and if he dies, then follow his funeral."