Source: Fatwa lil Imam an-Nawawi ´Al-Masa´il al-Manthurah" Page:50
Imam an-Nawawi was asked:
If you sneez during prayer, is it recommended that says: "Alhamdulillah"?
And if you say it, is it recommended for those who hear it say: "YarhamukAllah"?
It is recommended to say it, and it is also recommended for those who are not in the prayer saying: "YarhamukAllah" to him