This is so great, and so (rightly) well-loved, that I dearly hope it isn't apocryphal.
You can find it all over the internet via Google — yet never sourced! I searched my massive Logos library, and Spurgeon.org, and a good bit of my electronic Spurgeon collection. Nothing.
It's Spurgeon's remark — sometimes presented as a statement, sometimes as a response to a question — about the relationship of election to preaching the Gospel. Spurgeon (is reported as having said) that if God had painted a yellow stripe up the backs of the elect, he'd go around London lifting up coats and preaching only to them. As He has not (Spurgeon reportedly went on), Spurgeon would simply preach the Gospel to all, and let God bring in His own.
Can anyone site the source in Spurgeon?
UPDATE: well, we not having come up with a definitive answer, I've cast the net a bit wider.