This quote, like so many romantic quotes, gets piled on that already top-heavy stack of pseudo-Shakespeare lines. In fact, it is the work of Arrigo Boito, an Italian poet and librettist. This line, in its original Italian —
Come ti vidi
E tu sorridi
Perchè lo sai.
— is from his libretto for the 1893 opera Falstaff, with music by good ol’ Giuseppe Verdi. The opera was based on Shakespeare’s The Merry Wives of Windsor, so it’s not completely insane to think the line is the Bard’s as well. But it’s not. It’s from Act 2, Part 2, and is spoken by the character Fenton to Nanetta.
You’re still allowed to use it at your wedding. Just cite it correctly in your vows. (Footnotes are encouraged in “I do”s.)