Praise be to Allah.
It was proven in the hadeeth of Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah likes the act of sneezing and dislikes the act of yawning, so if any one of you sneezes and praises Allaah (says “al-hamdu Lillaah”), it is a duty on every Muslim who hears him to say to him, “Yarhamuk Allaah (may Allaah have mercy on you).” As for yawning, it is from the Shaytaan, so if any of you feels the urge to yawn, he should suppress it as much as he can, for when any one of you yawns, the Shaytaan laughs at him.”(Reported by al-Bukhaari, 10/505)
It was also reported from Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “If any one of you sneezes, let him say ‘Al-hamdu Lillaah,’ and let his brother or his companion say, ‘Yarhamuk Allaah.’ And if he says to him, ‘Yarhamuk Allaah,’ let him say, ‘Yahdeekum Allaahu wa yusliha baalakum (May Allaah guide you and correct your thinking).’”(Reported by al-Bukhaari, 10/502)
Anas (may Allaah be pleased with him) reported that two men sneezed in the presence of the Prophet (peace and blessings of Allaah be upon him), who said “Yarhamuk Allaah” to one of them, but not to the other. The one to whom he did not say it said: “So and so sneezed, and you said ‘Yarhamuk Allaah’ to him; I sneezed but you did not say it to me.” He said, “He praised Allaah (said ‘al-hamdu Lillaah’) and you did not.”(Reported by al-Bukhaari, 10/504)
Abu Moosa al-Ash’ari reported that the Prophet (peace and blessings of Allaah be upon him) said: “When one of you sneezes and praises Allaah, then say ‘Yarhamuk Allaah’ to him, but if he does not praise Allaah then do not say it.”(Reported by Muslim 2992)
Sneezing brings a person the blessing of relief by releasing vapours that were trapped in the head which, if they were to remain there, would cause him pain and sickness. For this reason, Islam tells him to praise Allaah for this blessing and for the fact that his body is still intact after this jolt that shook him like an earthquake… for sneezing causes a disturbing movement in the body… It was said (concerning the origin of the word “tashmeet” [which is translated here as “saying ‘yarhamuk Allaah’, but the original meaning is to tell someone to rejoice at another’s ‘misfortune’]): this is telling him to feel joyful because he has annoyed the Shaytaan by praising Allaah for the blessing of sneezing and its association with things that Allaah likes. When a person mentions Allaah and praises Him, this annoys the Shaytaan in several ways, for example: the act of sneezing itself, which Allaah likes; his praising Allaah for it; the Muslims praying for mercy for him; and his prayer for them to be guided. All of this annoys the Shaytaan and makes him upset, so the believer is told to rejoice over this annoyance of his enemy. Praying for mercy for the one who sneezes is called tashmeet because it implies that he should rejoice over the annoyance of his enemy. This is a good meaning which people should bear in mind when they sneeze or say ‘yarhamuk Allaah’ to one who sneezes, and they should appreciate the blessing of sneezing for the body and mind, and understand why Allaah likes it. Praise be to Allaah as is His due for He is the Most Generous and Majestic.
The scholar Ibn Muflih al-Hanbali (may Allaah have mercy on him) reported that Imaam Ibn Hubayrah said: “Al-Raazi reported from some doctors: sneezing is not a sign of the onset of sickness, unless it is accompanied by catarrh.” Ibn Hubayrah said: “If a person sneezes, this is an indication of good health, sound digestion and physical strength, so he should praise Allaah. This is why the Messenger of Allaah (peace and blessings of Allaah be upon him) commanded us to praise Allaah.”
It is also important to know that the Muslim should submit to the texts and act in accordance with them, without trying too hard to understand precisely why a text commands or forbids something. He should believe that Allaah is the Wise and Aware, Who doe not prescribe anything for His slaves unless it is in their best interests, in this world and the next, even though it may not cross their minds and some people may understand part of the wisdom behind it whilst others will never know. The Muslim’s basic rule is to follow the commands, and if he has some knowledge of the reason behind the commands, then praise be to Allaah.
And Allaah knows best.
For further details on the rules concerning sneezing, see: Al-Aadaab al-Shara’iyyah by Ibn Muflih, 2/334; Zaad al-Ma’aad by Ibn al-Qayyim, 2/438; Ghadhaa’ al-Albaab by al-Safaareeni, 1/441