When was hash chain first used? Hash linking is used to prove the integrity of a blockchain, or similar systems. When was that technique first used? I would guess it was early, maybe 1950s/1960s?
Lamport suggested the use of hash chaining in 1981 in Password Authentication with Insecure Communication, Communications of the ACM 24.11 (November 1981), pp 770-772.

He cites 3 prior papers:


Diffie, W., and Hellman, M.E. New directions in cryptography.
IEEE Trans. Inform. Theory 1T-22 (Nov. 1976), 644-654.
Evans, A., Kantrowitz, W., and Weiss, E. A user authentication
scheme not requiring secrecy in the computer. Comm. A CM 17, 8
(Aug. 1974), 437-442.
Wilkes, M.V. Time-Sharing Computer Systems. American
Elsevier, New York, 1972. 


[1] is the paper which essentially invented Public Key Cryptography in the open literature. Lamport refers to the use of a one way function F, as described there, as hash functions in his chain.

[2] and [3] are cited for "the widespread use of such a function", e.g., storing $y=F(x)$ instead of $x$.

So it seems to me Lamport may well be the first to suggest the use hash chaining. 

Edit: Thanks to @Gilles for pointing out Merkle patented hash trees in 1979.