Musnad Abu Ya'la al-Mawsili 3:249

Ibrahim ibn Sa'id al-Jawhari narrated to us, Yahya ibn Nasr ibn Hajib narrated to us, Hilal ibn Khabbab narrated to us, from Zadhan, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The right of the Muslim upon the Muslim is six: he greets him when he meets him, he answers him when he invites him, he gives him sincere advice in his absence, he says 'Yarhamuk Allah' (may Allah have mercy on you) to him when he sneezes, he visits him when he is ill, and he attends his funeral when he dies."