Musnad Ahmad 30:1152

Abu Abd al-Rahman narrated to us, Sa'id narrated to us, Abdullah ibn al-Walid narrated to us, from Ibn Hujayrah, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The right of the believer upon the believer is six qualities: that he greets him when he meets him, and he responds to him when he sneezes, and if he invites him, he answers him, and if he falls ill, he visits him, and if he dies, he attends his funeral, and if he is absent, he acts sincerely toward him."