The Prophet ﷺ said: Invoke a blessing on one who sneezes three times; (and if he sneezes more often), then if you wish to invoke a blessing on him, you may invoke, and if you wish (to stop), then stop.  

عَنِ النَّبِيِّ ﷺ قَالَ تُشَمِّتُ الْعَاطِسَ ثَلاَثًا فَإِنْ شِئْتَ أَنْ تُشَمِّتَهُ فَشَمِّتْهُ وَإِنْ شِئْتَ فَكُفَّ  

from his mother, from her father, who said: "The Messenger of Allah ﷺ said: 'Reply three times to the sneezing person. If he (sneezes) more, then if you wish reply and if you do not then do not.'"  

قَالَ رَسُولُ اللَّهِ ﷺ يُشَمَّتُ الْعَاطِسُ ثَلاَثًا فَإِنْ زَادَ فَإِنْ شِئْتَ فَشَمِّتْهُ وَإِنْ شِئْتَ فَلاَ  

"شَمِّتِ الْعَاطِسَ ثَلَاثًا؛ فَإِنْ زَاد فَإِنْ شِئْتَ فَشَمِّته وإن شئت فلا"