In my situation, I knew that I would be leaving San Diego and quitting my job many months in advance. I knew when we’d be leaving but I didn’t know where we’d be heading(since my fiancee was applying to med school). That really forced me to think outside of the box and come up with some unique ways to make money, independent of our future location. I could have sat back and hoped that she got into a school in a city where I could find work as an engineer but I didn’t want to rely on chance.
However, this comes back to the old discussion of pain versus pleasure. We will always do more to avoid pain than we will to gain pleasure. When our backs are against the wall, we act. When they're not, we relax. The truth is that the pain-versus-pleasure paradigm only operates in the short term. We'll only avoid pain in the here and now. Often not in the long term.
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.