2 more details because one-line snarky answers are bad
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You should read the original paper and then cite it.

Citations exist for many reasons.

  1. You should cite articles to show that you've done your due-diligence in learning the background information. Like the infamous Van Halen brown M&Ms, this is a quick way for your readers and reviewers to check that you actually know what you're talking about.

  2. You cite other articles to allow readers to find the relevant background material. Research papers create a web of information and providing signs pointing to the important previous work is important for those not immersed in your paper's field.

  3. Finally, you cite other work to give recognition and "props" to those who have done important work in the past. This is especially important to any work that you're actively building off of.

All three reasons I mention above are relevant to your question. This is your Master's thesis. You need to show that you've down the background reading, give an overview for future readers, and cover the important previous work. Sure you can cover secondary sources, but primary sources are better. If you're already familiar with the theorem and its proof, skimming the original paper shouldn't take much time and citing it is the more correct thing to do. Doubly so for a thesis.

You should read the original paper and then cite it.

You should read the original paper and then cite it.

Citations exist for many reasons.

  1. You should cite articles to show that you've done your due-diligence in learning the background information. Like the infamous Van Halen brown M&Ms, this is a quick way for your readers and reviewers to check that you actually know what you're talking about.

  2. You cite other articles to allow readers to find the relevant background material. Research papers create a web of information and providing signs pointing to the important previous work is important for those not immersed in your paper's field.

  3. Finally, you cite other work to give recognition and "props" to those who have done important work in the past. This is especially important to any work that you're actively building off of.

All three reasons I mention above are relevant to your question. This is your Master's thesis. You need to show that you've down the background reading, give an overview for future readers, and cover the important previous work. Sure you can cover secondary sources, but primary sources are better. If you're already familiar with the theorem and its proof, skimming the original paper shouldn't take much time and citing it is the more correct thing to do. Doubly so for a thesis.

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You should read the original paper and then cite it.