Respond three times to your brother when he sneezes, and if he sneezes more often, he has a cold in his head.  

شَمِّتْ أَخَاكَ ثَلاَثًا فَمَا زَادَ فَهُوَ زُكَامٌ  

I know him that he traced this tradition back to the prophet (May peace be upon him). Abu Dawud said: Abu Nu’aim transmitted it from Musa b. Qais, from Muhammad b. Ajlan, from Sa’id, on the authority of Abu Hurairah, from the prophet (May peace be upon him).  

أَنَّهُ رَفَعَ الْحَدِيثَ إِلَى النَّبِيِّ ﷺ بِمَعْنَاهُ  

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  

when a man sneezed beside the prophet (May peace be upon him), he said to him : Allah have mercy on you, but when he sneezed again, he said : The man has a cold in the head.  

أَنَّ رَجُلاً عَطَسَ عِنْدَ النَّبِيِّ ﷺ فَقَالَ لَهُ يَرْحَمُكَ اللَّهُ ثُمَّ عَطَسَ فَقَالَ النَّبِيُّ ﷺ الرَّجُلُ مَزْكُومٌ