Abu Hurairah (RAA) narrated that the Messenger of Allah (ﷺ) said:
“When one of you sneezes, he should say, ‘Alhamdulillah (Praise is to Allah) and his (Muslim) brother should say to him, ‘Yarhamuka Allah (May Allah have mercy on you). When he says this to him (the last phrase), he should reply, ‘Yahdikum Allah (May Allah guide you) and grant you well being.” Related by Al-Bukhari.
وَعَنْ اَلنَّبِيِّ ‏- صلى الله عليه وسلم ‏-قَالَ: { إِذَا عَطَسَ أَحَدُكُمْ فَلْيَقُلْ: اَلْحَمْدُ لِلَّهِ, وَلْيَقُلْ لَهُ أَخُوهُ يَرْحَمُكَ اَللَّهُ, فَإِذَا قَالَ لَهُ: يَرْحَمُكَ اَللَّهُ, فَلْيَقُلْ: يَهْدِيكُمُ اَللَّهُ, وَيُصْلِحُ بَالَكُمْ } أَخْرَجَهُ اَلْبُخَارِيُّ.‏ (1906)‏ .‏


‏2 ‏- صحيح.‏ رواه البخاري (6224)‏.‏

English reference : Book 16, Hadith 1490
Arabic reference : Book 16, Hadith 1446