Sneezing does not invalidate your prayer even if it's frequent.<br /> Note that if your forehead is taken away from the Turbah and you can keep it so that it does not fall on the Turbah again, you should do so and this is considered one Sujood, no matter you have said the Zikr or not. And if you can't keep your forehead and it falls on the Turbah, both will be considered one Sujood and if you have not said the Zikr it is a Mustahab precaution to say it.