It's from his Dallas memorial speech. A couple of other people have made memes about it too.
At first I was thinking maybe the people making fun of him were being too hard on him. I was willing to give him the benefit of the doubt, and think he was just using the phrase as a figure of speech, for effect.
But the more I thought about it, and the context, the more it bothered me. So I decided to make a meme with my thoughts on it.