Mustadrak al-Hakim — Hadith 7883

It is narrated from Abu Burdah bin Abi Musa that he said: I was with Sayyiduna Abu Musa (may Allah be pleased with him) while he was in the house of Umm al-Fadl. Umm al-Fadl sneezed and Abu Musa responded to her; then I sneezed and he did not respond to me. I went to my mother and told her, and when Abu Musa came to her, my mother said: My son sneezed in your presence and you did not respond to him, while a woman sneezed and you responded to her? He said: Your son sneezed but did not praise Allah, so I did not respond to him; and she sneezed and praised Allah, so I responded to her. I heard the Messenger of Allah (peace and blessings be upon him) say: "When one of you sneezes and praises Allah, respond to him; and if he does not praise Allah, do not respond to him." At this she said: You have done well, you have done well.
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.