Sahih Muslim 2992
Abu Burda reported:
I visited Abu Musa, as he was in the house of the daughter of Fadl b. 'Abbas. I sneezed but he did not respond to it (by saying): Allah may have mercy upon you. Then she sneezed and he (Fadl b. 'Abbas) said: May Allah have mercy upon you. I came back to my mother and informed her about it, and when he came to her she said: My son sneezed in your presence and you did not say:" Allah may have mercy upon you, and she sneezed and you said for her:" May Allah have mercy upon you." Thereupon he said: Your son sneezed but he did not praise Allah and I did not beg mercy of Allah for him and she sneezed and she praised Allah and so I said: May Allah have mercy upon you, as I heard Allah's Messenger (ﷺ) as saying: When any one of you sneezes he should praise Allah and the other should say: May Allah have mercy upon you, and if he does not praise Allah, no mercy should be begged for him.
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، وَمُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، - وَاللَّفْظُ لِزُهَيْرٍ - قَالاَ حَدَّثَنَا 
 الْقَاسِمُ بْنُ مَالِكٍ، عَنْ عَاصِمِ بْنِ كُلَيْبٍ، عَنْ أَبِي بُرْدَةَ، قَالَ دَخَلْتُ عَلَى أَبِي مُوسَى وَهْوَ فِي
 بَيْتِ بِنْتِ الْفَضْلِ بْنِ عَبَّاسٍ فَعَطَسْتُ فَلَمْ يُشَمِّتْنِي وَعَطَسَتْ فَشَمَّتَهَا فَرَجَعْتُ إِلَى أُمِّي
 فَأَخْبَرْتُهَا فَلَمَّا جَاءَهَا قَالَتْ عَطَسَ عِنْدَكَ ابْنِي فَلَمْ تُشَمِّتْهُ وَعَطَسَتْ فَشَمَّتَّهَا ‏.‏ فَقَالَ
 إِنَّ ابْنَكِ عَطَسَ فَلَمْ يَحْمَدِ اللَّهَ فَلَمْ أُشَمِّتْهُ وَعَطَسَتْ فَحَمِدَتِ اللَّهَ فَشَمَّتُّهَا سَمِعْتُ رَسُولَ
 اللَّهِ صلى الله عليه وسلم يَقُولُ ‏
"‏ إِذَا عَطَسَ أَحَدُكُمْ فَحَمِدَ اللَّهَ فَشَمِّتُوهُ فَإِنْ لَمْ يَحْمَدِ اللَّهَ
 فَلاَ تُشَمِّتُوهُ ‏"‏ ‏.‏