The Prophet (ﷺ) 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 Book 78, Hadith 247
← Previous Hadith The Prophet (ﷺ) ordered us to do seven (things) and forbade us from seven (other things): He ordered us to pay a visit to the sick, to follow funeral possessions, to say: May Allah be merciful to you to a sneezer, - if he says: Praise be to Allah, to accept invitation (invitation to a wedding banquet), to return greetings, to help the oppressed, and to help others to fulfill their oaths (provided it was not sinful). And he forbade us from seven (things): to wear golden rings or golden bangles, to wear silk (cloth), Dibaj, Sundus and Mayathir.

Sahih al-Bukhari Book 78, Hadith 246 Next Hadith → The Prophet (ﷺ) said, " If anyone of you sneezes, he should say 'Al-Hamduli l-lah' (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-llah", the former should say, 'Yahdikumul-lah wa Yuslih balakum' (May Allah give you guidance and improve your condition).

Sahih al-Bukhari Book 78, Hadith 248