Why hash function are used in digital signatures? In the lectures I see, hash functions are always used when we want to sign a document $d$. My question is why they are used in digital signatures? Maybe, because it is more easy and fast to calculate the signature when is used a "block" signature scheme? Do you know some formal paper or book where this is described? I need that for my thesis.
Bruce Schneier writes in Applied Cryptography (2nd ed., p. 38f):


  In practical implementations, public-key algorithms are often too inefficient to sign long documents. To save time, digital signature protocols are often implemented with one-way hash functions (...). Instead of signing a document, Alice signs the hash of the document.


The references Schneier cites are:


D. W. Davies and W. L. Price, "The Application of Digital Signatures Based on Public-Key Cryptosystems," Proc. of the Fifth International Computer Communications Conference, Oct 1980, pp. 525--530.
D. W. Davies and W. L. Price, "The Application of Digital Signatures Based on Public-Key Cryptosystems," National Physical Laboratory Report DNACS 39/80, Dec 1980.