The speech here is about what is to be said upon sneezing and yawning. Al Bukhari reported in his Sahih on the authority of Abu Hurayrah (may Allah be pleased with him) who narrated that the Prophet (peace be upon him) said: "Allah likes sneezing and dislikes yawning, so if someone sneezes and then praises Allah, then it is obligatory on every Muslim who heard him, to say: May Allah be merciful to you (Yar-hamuka-l-lah). But as regards yawning, it is from Satan, so one must try one s best to stop it, if one says: Ha when yawning, Satan will laugh at him." [Sahih Al Bukhari No. 6223].
The wisdom behind praising Allah at sneezing is that a sneezer -as Ibn Al Qayyim says:
Has gained a great blessing due to sneezing which is the exist of the steams congested in his head that if they exist, many diseases would inflict a person. So, it is permissible for a sneezer to praise Allah for this blessing which could blow away his organs after this hard shake of the body. So, praise be to Allah as should for the honor of His Face and the honor of His Glory. [See: Zad Al Ma ad (2/ 438 - 439)].
It was mentioned in the Hadith that Allah loves sneezing because it contains goodness for man and for the praising, gratitude, and invocation which a person shows thereafter.
As for yawning, Allah does not love it because it is done by Satan and because in most cases it is accompanied with the heaviness, fullness, and the inclination of the body to laziness. A Muslim is commanded to suppress his yawning as much as he can. It was reported in Sahih Al Bukhari and Sahih Muslim on the authority of Abu Hurayrah (may Allah be pleased with him) who said that the Messenger of Allah (peace be upon him) said: "Yawning is from Satan and if anyone of you yawns, he should suppress his yawning as much as possible, for if anyone of you (during the act of yawning) says: Ha, Satan will laugh at him." In the wordings of Imam Muslim: "So, when one of you yawns, he should try to restrain it as far as it lies in his power." [Sahih Al Bukhari No. 3289 and Sahih Muslim No. 2994].
As for the Prophet s saying: "he should suppress his yawning as much as possible," a person tries to shut his mouth upon yawning, but if he cannot do that, he should place his hand or part of his clothes on his mouth.
It is not proper for a Muslim to yawn opening his mouth without placing his hand or part of his clothes on his mouth because the shape will be ugly and Satan may enter into his mouth. Imam Muslim reported in his Sahih on the authority of Abu Sa id Al Khudry (may Allah be pleased with him) who narrated that the Messenger of Allah (peace be upon him) said: "When one yawns, he should put his hand over his mouth, otherwise the devil will enter." [Sahih Muslim No. 2995]. Seeking protection from Allah at yawning was not reported from the Prophet (peace be upon him), but if a Muslim remembers at yawning that it is because of Satan then seeks refuge with Allah, there is no harm as long as he does not consider it a Sunnah.
As for sneezing, the Sunnah came with a set of manners and great rulings which is desirable for a Muslim to keep to because it is part of the beauty and perfection of this Shari ah that meets all his needs in all conditions.
Al Bukhari reported in his Sahih on the authority of Abu Hurayrah (may Allah be pleased with him) who narrated that the Prophet (peace be upon him) said: "If anyone of you sneezes, he should say: Al-Hamdulillah (Praise be to Allah), and his (Muslim) brother or companion should say to him: Yar-hamuka-l-lah (May Allah bestow his Mercy on you). When the latter says: Yar-hamuka-l-lah", the former should say: Yahdikumul-lah wa Yuslih balakum (May Allah give you guidance and improve your condition)." [Sahih Al Bukhari No. 6224].
So, dear brother, look at this beauty and perfection to which the Shari ah called upon sneezing: Praising, mercy, and invocation. A sneezer praises Allah and those who hear him invoke Allah for him with mercy, and the other invokes Allah for him with guidance and reformation. So, how beautiful connection and tie this is! Islam made the invocation for a sneezer one of Muslim s rights upon his Muslim brothers. It was reported in books of authentic Hadiths from the Hadith of Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace be upon him) said: "There are six rights of a Muslim upon a Muslim: When you meet him, greet him; when he invites you, accept his invitation; when he seeks counsel, give him advice; when he sneezes and praises Allah, say to him: May Allah have mercy on you (Yarhamuk-Allah) ; when he is sick, visit him; and when he dies, follow his funeral procession." [Has been previously attributed to its sources].
Tashmit [saying: Yarhamkumullah] is invoking Allah for goodness, as if a person invokes Allah for him with firmness and obedience. It was said: The meaning is: May Allah keep you away from gloating. This Tashmit is entitled for those who praise Allah at sneezing, but those who do not praise Allah at sneezing do not deserve Tashmit. It was reported in Sahih Al Bukhari and Sahih Muslim on the authority of Anas (may Allah be pleased with him) who said: “Two persons sneezed in the presence of Allah s Messenger (peace be upon him). He responded to the one said: "Yarhamuk-Allah (may Allah have mercy on you)” and did not respond to the other. The latter said to him: "You invoked a blessing for this man but did not do so in my case. The Prophet (peace be upon him) replied: He praised Allah (i.e., he said Al-hamdu lillah ) but you did not." [Sahih Al Bukhari No. 6225 and Sahih Muslim No. 2991].