Musnad Ahmad 788:210

Al-Qasim ibn Malik Abu Ja'far narrated to us, 'Asim ibn Kulayb narrated to us, from Abu Burdah, who said: I entered upon Abu Musa in the house of the daughter of Umm al-Fadl. I sneezed, and he did not say "Yarhamuk Allah" (may Allah have mercy on you) to me. Then she sneezed, and he said "Yarhamuk Allah" to her. I returned to my mother and informed her. When he came to her, she said: "My son sneezed in your presence and you did not say 'Yarhamuk Allah' to him, but she sneezed and you said 'Yarhamuk Allah' to her." He said: "Your son sneezed and did not praise Allah the Exalted, so I did not say 'Yarhamuk Allah' to him. She sneezed and praised Allah the Exalted, so I said 'Yarhamuk Allah' to her. I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'If one of you sneezes and praises Allah, then say "Yarhamuk Allah" to him; but if he does not praise Allah the Exalted, then do not say "Yarhamuk Allah" to him.'" She said: "You have done well. You have done well."