Abu Dawud
General Behavior
Book Number: 036 / Hadith Number: 5018
Narated By Ubayd ibn Rifa'ah az-Zuraqi : The Prophet (pbuh) said: Invoke a blessing on one who sneezes three times; (and if he sneezes more often), then if you wish to invoke a blessing on him, you may invoke, and if you wish (to stop), then stop.