Narrated 'Umar bin Ishaq bin Abi Talhah:
from his mother, from her father, who said: "The Messenger of Allah (ﷺ) said: 'Reply three times to the sneezing person. If he (sneezes) more, then if you wish reply and if you do not then do not.'" (JA)
English reference (JA): Vol. 5, Book 41, Hadith 2744 (JA)
Arabic reference (JA): Book 43, Hadith 2968 (JA)