According to Aish.com,

The story is told that Napoleon was walking through the streets of Paris one Tisha B'Av. As his entourage passed a synagogue he heard wailing and crying coming from within; he sent an aide to inquire as to what had happened. The aide returned and told Napoleon that the Jews were in mourning over the loss of their Temple.

Napoleon was indignant! "How come I wasn't informed? When did this happen? Which Temple?"

The aide responded, "They lost their Temple in Jerusalem on this date 1,700 years ago."

Napoleon stood in silence and then said, "Certainly a people which has mourned the loss of their Temple for so long will survive to see it rebuilt!"

Alternate versions have the story taking place in Acre, or Napoleon himself going into the synagogue; alternate punchlines have him saying, "I vow that this people is destined for a future in their own homeland. For is there any other people who have kept alive similar mourning and hope for so many years?" or "any people that still feel strongly about events that occurred to them thousands of years ago, will surely survive as a people, and persevere."

I have no idea how or where this story originated. Anyone?