I went in to see Abu Musa at the house of the daughter of Umm Fadl. I sneezed and he did not invoke a blessing on me, but she sneezed and he invoked a blessing on her. So I returned to my mother and told her about it.When he came to her, she said, "My son sneezed in your presence and you did not invoke a blessing on him, yet she sneezed and you did invoke a blessing on her."He replied, "Your son sneezed but did not praise Allah Almighty, so I did not invoke a blessing on him. She sneezed and praised Allah Almighty, so I invoked a blessing on her. I heard the Messenger of Allah ﷺ say, 'When one of you sneezes and praises Allah, then invoke a blessing on him, but if he does not praise Allah , do not invoke a blessing on him.'"She said, "Well done, well done."
حدثنا القاسم بن مالك أبو جعفر حدثنا عاصم بن كليب
دخلت على أبي موسى في بيت ابنة أم الفضل فعطست ولم يشمتني وعطست فشمتها فرجعت إلى أمي فأخبرتها فلما جاءها قالت عطس ابني عندك فلم تشمته وعطست فشمتها فقال إن ابنك عطس فلم يحمد الله تعالى فلم أشمته وإنها عطست فحمدت الله تعالى فشمتها وسمعت رسول الله ﷺ يقول إذا عطس أحدكم فحمد الله فشمتوه وإن لم يحمد الله عز وجل فلا تشمتوه فقالت أحسنت أحسنت
Its isnad is sahih. Asim bin Kulayb is trustworthy, and his hadith is included by Muslim. The hadith was narrated by Muslim, 4/2292, no. 2992, in the chapter on Zuhd, "Responding to the sneezer", and by Ibn Abu Shayba, 8/496, in the chapter on Adab, with similar wording, and by Hakim, 4/265, and Dhahabi agreed with him.