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 Messenger of Allah 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.  

عَطَسَ عِنْدَكَ ابْنِي فَلَمْ تُشَمِّتْهُ وَعَطَسَتْ فَشَمَّتَّهَا فَقَالَ إِنَّ ابْنَكِ عَطَسَ فَلَمْ يَحْمَدِ اللَّهَ فَلَمْ أُشَمِّتْهُ وَعَطَسَتْ فَحَمِدَتِ اللَّهَ فَشَمَّتُّهَا سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ إِذَا عَطَسَ أَحَدُكُمْ فَحَمِدَ اللَّهَ فَشَمِّتُوهُ فَإِنْ لَمْ يَحْمَدِ اللَّهَ فَلاَ تُشَمِّتُوهُ  

[Machine] I entered the house of Abu Musa to visit the daughter of Umm Al-Fadl. I sneezed, but she did not say "May Allah have mercy on you." Then she sneezed and I said "May Allah have mercy on you." I returned to my mother and informed her about it. When Abu Musa came to her, she said to him, "Your son sneezed in front of me but I did not say 'May Allah have mercy on you,' and then I sneezed and he said 'May Allah have mercy on you.'" Abu Musa said, "Indeed, your son sneezed and you did not praise Allah, so I did not say 'May Allah have mercy on you.' And when you sneezed and praised Allah, I said 'May Allah have mercy on you.'" Then I heard the Messenger of Allah, ﷺ , saying, "When any of you sneezes and praises Allah, you should say 'May Allah have mercy on you,' and if he does not praise Allah, do not say it." She said, "You did well, you did well."  

دَخَلْتُ عَلَى أَبِي مُوسَى فِي بَيْتِ ابْنَةِ أُمِّ الْفَضْلِ فَعَطَسْتُ وَلَمْ يُشَمِّتْنِي وَعَطَسَتْ فَشَمَّتَهَا فَرَجَعْتُ إِلَى أُمِّي فَأَخْبَرْتُهَا فَلَمَّا جَاءَهَا قَالَتْ عَطَسَ ابْنِي عِنْدَكَ فَلَمْ تُشَمِّتْهُ وَعَطَسَتْ فَشَمَّتَّهَا فَقَالَ إِنَّ ابْنَكِ عَطَسَ فَلَمْ يَحْمَدِ اللهَ تَعَالَى؛ فَلَمْ أُشَمِّتْهُ وَإِنَّهَا عَطَسَتْ فَحَمَدَتِ اللهَ تَعَالَى فَشَمَّتُّهَا وَسَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ إِذَا عَطَسَ أَحَدُكُمْ فَحَمِدَ اللهَ فَشَمِّتُوهُ وَإِنْ لَمْ يَحْمَدِ اللهَ ﷻ فَلَا تُشَمِّتُوهُ فَقَالَتْ أَحْسَنْتَ أَحْسَنْتَ