Praise be to Allah.
It was proven in the hadeeth of Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Allah likes the act of sneezing and dislikes the act of yawning, so if any one of you sneezes and praises Allah (says “al-hamdu Lillaah”), it is a duty on every Muslim who hears him to say to him, “Yarhamuk Allah (may Allah 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 Allah 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 Allah.’ And if he says to him, ‘Yarhamuk Allah,’ let him say, ‘Yahdeekum Allahu wa yusliha baalakum (May Allah guide you and correct your thinking).’”
(Reported by al-Bukhaari, 10/502)
Anas (may Allah be pleased with him) reported that two men sneezed in the presence of the Prophet (peace and blessings of Allah be upon him), who said “Yarhamuk Allah” 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 Allah’ to him; I sneezed but you did not say it to me.” He said, “He praised Allah (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 Allah be upon him) said: “When one of you sneezes and praises Allah, then say ‘Yarhamuk Allah’ to him, but if he does not praise Allah 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 Allah 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 Allah’, 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 Allah for the blessing of sneezing and its association with things that Allah likes. When a person mentions Allah and praises Him, this annoys the Shaytaan in several ways, for example: the act of sneezing itself, which Allah likes; his praising Allah 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 Allah’ to one who sneezes, and they should appreciate the blessing of sneezing for the body and mind, and understand why Allah likes it. Praise be to Allah as is His due for He is the Most Generous and Majestic.
The scholar Ibn Muflih al-Hanbali (may Allah 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 Allah. This is why the Messenger of Allah (peace and blessings of Allah be upon him) commanded us to praise Allah.”
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 Allah is the Wise and Aware, Who does 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 Allah.
And Allah 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