A. M. Turing. On Computable Numbers, with an Application to the Entscheidungs problem. Proceedings of the London Mathematical Society. 2 (1937) 42: 230–265. (pdf)

A. N. Kolmogorov and V. A. Uspénski. On the definition of an algorithm. Uspehi Mat. Nauk. 13 (1958), 3-28. English translation in American Mathematical Society Translations, Series II, Volume 29 (1963), pp. 217–245. (math-net.ru). Also see JSL review by Elliott Mendelson on jstor.