Ibn Mas'ud reported that the Prophet, may Allah bless him and granthim peace, said, "There are four things that one Muslim owes another Muslim:
to visit him when he is ill, to be with him when he dies, to accept whenhe invites him, and to ask for mercy on him when he sneezes."
English reference: Book 40, Hadith 923
Arabic reference: Book 1, Hadith 923