Is it permissible to say "Alhamdulillah" when one sneezes during prayer, and for the worshipper behind him to respond with "Yarhamukallah"?

It is permissible for someone who sneezes during prayer to say "Alhamdulillah" (Praise be to Allah) silently, according to the sounder opinion. However, if someone responds to the sneezer by saying "Yarhamukallah" (May Allah have mercy on you) during prayer, their prayer becomes invalid, because it is speech directed at a human being and is part of people's everyday communication.