It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Respond to the one who sneezes (when he says 'Alhamdulillah') with 'Yarhamuk Allah' (May Allah have mercy on you) three times; then if he sneezes again, it is a cold."
Imam al-Tabarani (may Allah have mercy on him) said: "This hadith was narrated from al-Zuhri (may Allah have mercy on him) only by Yahya bin Abi Unaysah (may Allah have mercy on him)."
Hadith Referenceالمعجم الأوسط / باب الميم / 8899
Hadith Takhrij«أخرجه أبو داود فى «سننه» برقم: 5034، (4/467) [دون رقم في الترقيم المختار]، والبزار فى «مسنده» برقم: 8498»