Don't mistake passive income with zero work. It's still work, it's just that your income is not directly tied to the hours worked. Anyone who owns rental properties knows that it's considered passive income but there is quite a bit of work involved. The work is front heavy but if you are lucky, you can collect rental checks without incident for many months before having to do work.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.