Hadith 5034
Chapter 100: How many times should one say "May Allah have mercy on you" to one who sneezes? - كتاب الأدب
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنِ ابْنِ عَجْلاَنَ، قَالَ حَدَّثَنِي سَعِيدُ بْنُ أَبِي سَعِيدٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ شَمِّتْ أَخَاكَ ثَلاَثًا فَمَا زَادَ فَهُوَ زُكَامٌ .
Narrated AbuHurayrah:
Respond three times to your brother when he sneezes, and if he sneezes more often, he has a cold in his head.
Reference : Sunan Abi Dawud 5034
In-book reference : Book 43, Hadith 262