A Lincoln quote

[STS Lincolnite]

While I have a few minutes (I am away from the house for a bit), I am online and trying to track down a quote that Lincoln made regarding assassination attempts. It goes something like this:

"If someone wants to get at me, there is nothing I can do to prevent it" or perhaps "If someone truly wants to do me harm, there is nothing I can do to prevent it," or any of a number of other variations I have come across.

These variations are put forth by various non-historians and historians alike as I am looking online. None of what I have found so far has a cited source. I will be digging into this a little more when I get a chance but if anyone can point me in the right direction to finding the actual quote and then a citation for it, I would be very grateful.

Thanks!