Replying to the person who sneezes [by saying yarhamakAllah] is it Fardh 'ayn or Fardh kifaayah?
On the auhtority of Aboo Hurairah - may Allah be pleased with him - [he said] that the Messenger of Allah said:
The rights of a muslim upon his fellow muslim are five. Replying the salaam, visiting the sick, following the funeral and replying to the person who sneezes [by saying yarhamakAllah] [Reported by Bukhaaree, Kitaabal-Janaa-iz no. 1240 and Muslim, Kitaab al-Salaam no. 21626]
Explaining this hadeeth Shaykh ibn 'Uthaymeen - may Allah have mercy upon him - said:
If the person who sneezes [remembers to] praise[s] Allah then is it Fardh 'ayn [meaning obligatory upon every person] or is it Fardh kifaayah [meaning obligatory on the muslim community only as a whole] to reply to such a person [by saying may Allah have mercy upon you]? Meaning is it sufficient for one person in the muslim community to reply to a sneezing person or is it obligatory upon every single person who hears the sneezing person to reply?
[And the answer is] Some of the scholars have mentioned this to be fardh kifaayah, so if we were a group of people and one amongst us sneezed, then it would be sufficient for one person amongst us to reply to this individual.
Whilst others [from amongst the scholars] have said, rather replying to the sneezing person is fardh 'ayn upon every person that hears the sneezing person, because the Prophet said it is the right upon every person that hears him sneeze to reply by saying yarhamakAllah [may Allah have mercy upon you] so what is evident is that it is fardh 'ayn. So based upon this, [we say] that everyone that hears a person sneeze it is obligatory upon him to say yarhamakAllah and the sneezing person replies by saying yahdeekum-ul-Allahu wa yuslih baalakum [may Allah guide you and make your affairs good] and it is sufficient for him to say this to one person if he makes the intention for the whole group.