People tend to take everything they ever hear that sounds romantic or sounds wise and they say that Shakespeare said it. I do not believe that this is a quote from Shakespeare. Instead, I believe that it is a quote from a poem called "Bedouin Song" by Bayard Taylor.
I have checked in databases that include all of Shakespeare's plays and poetry and he never wrote those words. In fact, he never even wrote the words "I love thee." Strange, huh?
Anyway, I also have a link to the Taylor poem -- check it out.
Yep - Bedouin Song, although it is regularly printed on things from greeting cards to mugs to dishtowels and attributed to WS. Weird.