Fatwa no. 16903
Q: If a person sneezes while offering salah (Prayer), should they say “Alhamdu lillah [All praise is due to Allah]”? Also, if a person yawns while offering Salah, do they have to say Isti`adhah (saying: “A`udhu-Billahi mina Al-Shaytan -ir-Rajim [I seek refuge with Allah from the accursed Satan]”)?
A: If a person offering Salah sneezes, they should say, “Alhamdu lillah” in a low voice; because there are authentically reported Hadith from the Prophet (peace be upon him) that indicate the permissibility of saying so.
As for Isti`adhah after yawning, there is no origin for it in the Shari`ah (Islamic law). However, the person who yawns should hold themselves from yawning as much as they can, but there is no problem if they say Isti`adhah while yawning in or outside Salah.