Sneezing During Friday Sermon

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.

End quote. 

Al-Mirdaawi (may Allah have mercy on him) said: It is permissible for him to say Ameen to the du‘aa’ 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.