noun
Refers to the origin or means by which one earns money, such as a job, investment, or business.
彼の主な収入源は株式投資だ。
His main source of income is stock investment.
収入 means 'income' itself, while 収入源 specifies the 'source' of that income.
Compound of 収入 (income) and 源 (source).