Saying “Yarhamuk Allah (May Allah have mercy on you)” to one who sneezes

He does not deserve that, and it is makrooh to say “Yarhamuk Allah” to him. It was reported in the two Saheehs of al-Bukhaari and Muslim that Anas (may Allah be pleased with him) said:

‘Two men sneezed in the presence of the Prophet (peace and blessings of Allah be upon him). He said “Yarhamuk Allah” to one of them and 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, and I sneezed but you did not say it to me.” The Prophet (peace and blessings of Allah be upon him) said: “He said ‘Al-hamdu Lillaah’, and you did not.”

According to Saheeh Muslim, Abu Moosa al-Ash’ari (may Allah be pleased with him) said: “I heard the Messenger of Allah (peace and blessings of Allah be upon him) say, ‘When one of you sneezes and says “al-hamdu Lillaah”, then say “Yarhamuk Allah” to him, but if he does not say “al-hamdu Lillaah”, then do not say “Yarhamuk Allah” to him.’”

In Saheeh al-Bukhaari it is reported from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “When any one of you sneezes, let him say ‘Al-hamdu Lillaah’ and let his brother or companion say to him, ‘Yarhamuk Allah.’ If he says ‘Yarhamuk Allah’ to him, let him say, ‘Yahdikum Allahu wa yusliha baalakum (May Allah guide you and reform your thinking).’”