If a person who is listening to the khutbah (sermon) on Friday sneezes, he should say al-hamdu Lillah (praise be to Allah) quietly to himself. If anyone hears him, it is not permissible for him to say to him Yarhamuk Allah (may Allah have mercy on you).
It says in al-Mudawwanah (1/230): Maalik said concerning someone who sneezes when the imam is delivering the khutbah that he should say al-hamdu Lillah quietly to himself. No one should say Yarhamuk Allah to the one who sneezed when the imam is delivering the khutbah.
Al-Mirdaawi (may Allah have mercy on him) said: It is permissible for him to say Ameen to the duaa' and to say al-hamdulillah silently if he sneezes. End quote from al-Insaaf, 2/418
Shaykh Ibn Baaz (may Allah have mercy on him) said: If he sneezes, he has to praise Allah (say al-hamdu Lillah) to himself and not raise his voice.
End quote from Majmoo al-Fataawa, 12/411
Shaykh Ibn Uthaymeen (may Allah have mercy on him) said: If a member of the congregation sneezes on Friday, he should say al-hamdulillah silently. If he says it out loud and the people around him hear him, it is not permissible for them to say Yarhamuk Allah to him.
End quote from ash-Sharh al-Mumti, 5/109
And Allah knows best.
Reproduced from Islam QA