The Rights of a Believer on a Fellow Believer.
Kitāb al-Muʾmin|Volume 1|Book 1|Chapter 4
99 - وعنه (ع) قال إنّ مِن حقِّ المسلمِ إن عَطسَ أنْ يُسمّتَه وإنْ أولِمَ أتاه وإنْ مرِضَ عادَه وإنْ ماتَ شهِِدَ جَنازتَه.
99. It has been reported that Abu 'Abd Allah عليه السلام said: Amongst the rights of a Muslim is that when he sneezes, Allah's Name be invoked (to bless him); if he is in pain, he should be relieved of it; if he is sick, he should be visited; and if he dies, his bier should be accompanied.