Citing propositions etc. in a master's thesis
Including propositions from elsewhere with just a generic remark at the start of the chapter is risky, and I would strongly advise against it. 

In particular when it comes to proofs this makes it very difficult to see what is going on: Are you providing your own proofs for statements deemed to be "obvious" in the paper? Are you expanding a proof from the source? Are you merely paraphrasing?

For propositions, a rather elegant way that does not impact the writing flow would be:

\begin{proposition}[\cite[Proposition XX]{source}]

...

\end{proposition}

For proofs, you can just put a sentence outlining the extent of your contribution at the start, where it also does not break the flow.