Abu Salih reported that the Prophet, may Allah bless him and granthim peace, said, "When one of you sneezes, he should say, 'Praise be toAllah.' When he says, 'Praise be to Allah, ' his brother or companion shouldsay to him, 'May Allah have mercy on you.' Then let the person who sneezedsay, 'May Allah guide you and put your affairs in order.'"
English reference: Book 40, Hadith 927
Arabic reference: Book 1, Hadith 927