When should I introduce a reference? I am currently writing my research proposal and I have a piece where I quote a book. In summary I am saying the following:


  Mr. X says that there is a very high probability that squares have four sides. This can be concisely summarised as "a polygon with four sides". ... 


Note that at the end I am quoting him directly. 

When do I put a citation to Mr. X’s book?


Immediately after the first sentence.
At the end, when I stop referring to him in the paragraph, i.e., after the second sentence or later.
Both?
Disclaimer: This is my opinion

I can think of a couple ways: 

First, edit the first sentence so that it does not require a reference:


  It has been postulated that squares are very probable to have four
  sides. For instance, Mr. X concisely summarised that ‘a polygon with
  four sides’... (Ref)


Second, chain the sentences together, eliminating the period:


  Mr. X, a main proponent who suggests that there is a very high
  probability that squares have four sides, concisely summarised that ‘a
  polygon with four sides’... (Ref)


Or


  Mr. X says that there is a very high probability that squares have
  four sides: he summarised that ‘a polygon with four sides’... (Ref)


Third, keep the current structure.

I think it's acceptable to cite only once if you can keep the subjects consistent:


  Mr. X says that there is a very high probability that squares have
  four sides. He concisely summarised that ‘a polygon with four sides’...
  (Ref)


The fact that the first subject is "Mr. X" and then the second subject became "This (idea)" causes the disjoint feeling. Unifying the subjects will leave no room for misinterpretation.