"Bless You"
Why? Why are these words spoken ?
The words are spoken because Wishing someone well after they sneeze probably originated thousands of years ago. The Romans would say "Jupiter preserve you" or "Salve," which meant "good health to you," and the Greeks would wish each other "long life." The phrase "God bless you" is attributed to Pope Gregory the Great, who uttered it in the sixth century during a bubonic plague epidemic (sneezing is an obvious symptom of one form of the plague).

when? When did this saying originated ?
it started during the plague of AD 590.

How? how did this saying originated ?
 pope Gregory thought everyone be blessed after sneezing because sneezing was an obvious symptom of plague. 

where? Where did this saying originated?
Rome