Source; the origin or starting point of something (e.g., information, funds, a river, or a cause).
この川の源はあの山の雪解け水だ。
The source of this river is the meltwater from that mountain.
情報の源を必ず確認してから記事を書くべきだ。
You should always verify the source of information before writing an article.
彼の不安の源は仕事のプレッシャーだった。
The source of his anxiety was the pressure of his job.