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." (FR)
English reference (FR): Book 40, Hadith 923 (FR)
Arabic reference (FR): Book 1, Hadith 923 (FR)