From Richard Powers’ moving science fiction story Genie, available as a Kindle Single (Amazon.com, Amazon UK).
Let us assume that there is, indeed, sentient life in one or another part of remote space What on earth are we going to talk about? Hello, are you there?, from us, followed by Yes, hello, from them–will take two hundred years at least Perhaps the safest thing to do at the outset, if technology permits, is to send music. This language may be the best we have for explaining what we are like to others in space, with least ambiguity. I would vote for Bach, all of Bach, streamed out into space, over and over again. We would be bragging, of course, but it is surely excusable for us to put the best possible face on at the beginning of such an acquaintance. We can tell the harder truths later.
See also my review of Richard Powers’ novel Orfeo.
Update: As Laurent mentions below, this is a comment from The Lives of a Cell by Lewis Thomas. Looking back at the story, I see it is cited as such. (I had highlighted it on my Kindle, and grabbed the quote from my Kindle page, without being able to see the context.) Great quote nevertheless.