This may help provide a lead. The subject is Grant, of course. However, while this site provides the quotation, a source isn't given. Try looking to see if the below citation is useful.

Best of luck!

http://www.bright.net/~lrrp/spotch.html

The next day, May 19th, Lee's demonstration against Grant's right resulted in severe fighting. Grant
withdrew 2 days later, once more endeavoring to move beyond Lee's right. "We have met a man, this
time," a Confederate wrote of Grant, "who either does not know when he is whipped, or who cares not if
he loses his whole Army."

Historical Times Illustrated Encyclopedia of the Civil War