It is permissible to say “Alhamdolillah” – all praise belong to Allah – after sneezing during Salat.

In the following Hadith, the Prophet^(sa) praised a companion who praised Allah after sneezing during Salat.

Hazrat Rifa’ah^(ra) bin Rafi said:

“I prayed behind God’s Messenger, and on sneezing, I said, “Praise be to God, great, good, blessed and abounding in good, as our Lord wishes and is pleased with.” When God’s Messenger finished his prayer he moved away and asked, “Who was it who spoke during the prayer?” but no one replied. He asked a second time, but no one replied. He asked a third time, and Rifa‘ah said, “I did, Messenger of God.” Thereupon the Prophet said, “By Him in whose hand my soul is, over thirty angels raced to it to see which of them would ascend with it.” (Mishkat al-Masabih, What may not be done during Prayer and what is permitted – Section 2)