"On Computable Numbers, with an Application to the EntscheidungsProblem", by AlanTuring.
Proceedings of the London Mathematical Society, Series 2, Vol.42 (1936 - 37) pages 230 to 265, with corrections from Proceedings of the London Mathematical Society, Series 2, Vol.43 (1937) pages 544 to 546.
<http://www.abelard.org/turpap2/tp2-ie.asp>
This is the paper that defined the notion of TuringMachine (referred to as an "automatic machine" or "a-machine").