Praise be to Allah,
Ismaa’eel [Ibraaheem ibn ‘Ulayyahj narrated to us from Ismaa’eel [ibn Iarkhaan] at-Taymee from Anas ibn Maalik who said,
“Two men sneezed in the presence of the Prophet ( peace and blessings of Allah be on him) and he said ‘Yarhamuk-Allaah’ (may Allah have mercy on you) to one of them but not to the other. It was said [to him], ‘Two men sneezed, and you said Yarhamuk-Allaah to one of them but not to the other. Why is that?’ He replied, ‘This one praised when he sneezed, but the other one did not praise Allah.”‘
(Musnad Ahmad, Vol. 3, p. 176. The isnaad of this hadeeth is Saheeh. according to the conditions set by al-Bukhaaree and Muslim.)
Commentary
When one sneezes, one should praise Allah by saying ‘Al-Hamdu lillaah ‘. In reply to this, one who hears the person who has sneezed say this should say to him, ‘Yarhamuk-Allaah’ (may Allah have mercy on you). The person who has sneezed must reply by saying, ‘Yahdeekum-ullaahu wa yuslihu baalakum’ (May Allah guide you and improve your condition).
Source:
Thulaathiyyaat Musnad Ahmad ibn Hanbal (may Allah have mercy on him)
Reports of Imaam Ahmad that have only three narrators between him and the Prophet, peace, and blessings be upon him.
Shaykh Muhammad ibn Ahmad ibn Saalim as–Saffaareenee al–Hanbalee ~(1114 –1188 AH)