Abu Burda said, "I came to Abu Musa while he was in the house ofUmm al-Fadl ibn al-'Abbas and I sneezed and Abu Musa did not wish mercyon me. Umm al-Fadl sneezed and he wished mercy on her. I told my mother,so when Abu Musa came to her, she confronted him and said, 'My son sneezedand you did not wish mercy on him. She (Umm al-Fadl) sneezed and you wishedmercy on her.' Abu Musa replied, 'I heard the Prophet, may Allah blesshim and grant him peace, say, "When one of you sneezes and praises Allah,you should wish mercy on him, if he does not praise Allah, do not wishmercy on him." My son sneezed and did not praise Allah, so I did not wishmercy on him. She sneezed and praised Allah, so I wished mercy on her.'She said, 'You did well.'"
English reference: Book 40, Hadith 941
Arabic reference: Book 1, Hadith 941