Musnad Ahmad 30:1724

Sulayman narrated to us, he said: Isma'il narrated to us, he said: Al-'Ala' informed me, from his father, from Abu Hurayrah, that the Prophet (peace and blessings of Allah be upon him) 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, greet him; if he invites you, answer him; if he seeks your advice, advise him; if he sneezes and praises Allah, say 'Yarhamuk Allah' (may Allah have mercy on you) to him; if he falls ill, visit him; and if he dies, follow his funeral."