I wouldn't think of a high yield savings account as a source of passive income but your savings should be getting something (less like Seinfeld syndication residuals and more like a commercial jingle residuals!). It won't make you rich but it's nice if your baseline, risk-free rate of return on cash is 1% or more. The best high yield savings accounts (or money market accounts) offer higher interest rate and there is absolutely no risk. CIT Bank currently leads the pack with the highest interest rate.
Investing in bonds: Similarly, bonds are an attractive way to engage in passive income. Over a recent 45-year period, bonds funds, as measured by Vanguard Funds, returned 7.1%. Of course, there's no guarantee that investments in stocks or bonds will always work out well, investing in them is by far the surest way to generate money through passive income.

Portfolio income can come from multiple sources – interest/bond coupons, stock dividends, financial strategies including derivatives and capital growth.  Each offers some cash flow and some also offer potential capital gain with some risk from liquidity and volatile prices.  In this low-yield environment, many investors rely too heavily on cash flows and to pursue this are venturing further into riskier areas than they’d normally consider.