Truly reliable dividend stocks can be hard to find, but there are a few companies on the market that are great candidates for income investors. If you're looking for long-term passive income from a stock, the company should possess sustainable competitive advantages, strong efficiency ratios, sufficient financial health, and a history of cash flow generation.

One of the leaders in the human resources software industry actually fits that description -- it's a passive income powerhouse thanks to decades of dependable cash flow and a stable outlook.

Stability is a prerequisite for unstoppable dividend stocks, and Automatic Data Processing (NASDAQ: ADP) is a model of stability with plenty of data to support that fact. The company is among the leaders in the payroll and human capital management software industries. It has customers in 140 countries, with more than 1 million clients. That diverse exposure means that ADP isn't tied to the well-being of any one customer, industry, or nation.

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Source Fool.com