The Lincoln Hunters by Wilson Tucker HB DJ - Science Fiction Book Club
"Benjamin Steward was violating a law of nature.
For the second time he was visiting a place and a period where he had been before, and men of the 26th century knew that no object could twice return to a place in time without "cancelling itself out."
Steward had been sent back 700 years to record a speech by Abraham Lincoln, and, because of an error on the part of the engineers who ran the time machine, it seemed likely he would be destroyed."