What is the ruling on praising Allah after sneezing and saying "Yarhamukallah" to the sneezer in Islam? Is it legislated by the Prophet, peace and blessings be upon him, or was it affirmed from previous laws? Did scholars agree on its legitimacy? Is it established by consensus or by individual reports? Is there a comprehensive book on this issue?

Sneezing and then praising Allah Most High (al-hamd) is a recommended act (mustahabb). Responding to the sneezed person with a supplication (tashmit) is an obligation (wajib) upon everyone who hears it. Some say it is a collective obligation (wajib kifai). This is supported by the hadith in Al-Bukhari and Muslim: "If one of you sneezes, he should say, 'Al-hamdu Lillah (Praise be to Allah),' and his brother or companion should say to him, 'Yarhamukallah (May Allah have mercy on you).' " The sneezed person should reply by saying, "Yaghfirullahu lana wa lakum (May Allah forgive us and you)" or "Yahdikumullahu wa yuslihu balakum (May Allah guide you and rectify your affairs)." Its legitimacy is agreed upon by scholars, and its authenticity from the Prophet, peace and blessings be upon him, is beyond doubt.