Question
What is the ruling on the prayer of someone who responds to a sneezer with 'May Allah have mercy on you' while in prayer?
Answer
Responding to a sneezer with 'May Allah have mercy on you' invalidates the prayer; because it involves engaging in conversation, and speaking during prayer invalidates it. See: Durar al-Hukkam 1: 102, and Fath Bab al-Inayah 1: 303.