Musnad Ahmad 980:4

Yahya ibn Sa'id narrated to us, from 'Abd al-Hamid ibn Ja'far, my father narrated to me, from Hakim ibn Aflah, from Abu Mas'ud, from the Prophet, peace and blessings of Allah be upon him, who said: "The Muslim has four duties toward the Muslim: that he answers him if he calls him, that he says 'Yarhamuk Allah' (may Allah have mercy on you) to him if he sneezes, that he visits him if he falls ill, and that he attends his funeral if he dies."