Not every sentence in the introduction requires a separate reference. Citations are essential for statements of fact, statistics, or prior research findings, but not for generally accepted knowledge or your own reasoning. Aim to cite key, high-quality sources that support the context, justify the research gap, and highlight the novelty of your proposal. Over-citation can clutter the text, while under-citation may weaken credibility.
The general approach to avoiding plagiarism is this. If it is entirely your idea, you don't have to provide a citation. If it is common knowledge, you don't have to provide a citation. If it is someone else's idea, you need to give them credit. If there is any question, provide a citation.
In research wise, Any fact statements does not need reference or citation (e.g., Sun is shining at daytime). well, That is obvious.
Any truth statements needs reference,evidence or citation (e.g., Sun is shining at night time!) well, who side so? where this idea come from? any references, citations?
In-text citations prevent plagiarism. Additionally, explanation should be provided how the cited literature supports the research topic, research questions, and objectives.