How many times should one say "May Allah have mercy on you" to one who sneezes?
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.