Musnad Ahmad 30:2218

'Affan narrated to us, he said: 'Abd al-Rahman ibn Ibrahim al-Qass narrated to us, he said: al-'Ala' ibn 'Abd al-Rahman narrated to us, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The right of the Muslim upon the Muslim is six." They said: "And 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, 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 accompany him (to his funeral)."