Passive income has been talked to death. MMM has written extensively about his investment strategy and his rental properties, as have every other FIRE blogger out there.
That being said most "passive" income streams aren't very passive. Even MMM spends a lot of time working on his properties. Real estate and being a landlord are not passive at all unless you hire a management company and a handyman to do the work for you. That eats into your bottom line, obviously. The same is true for being a business owner of any kind. No business runs itself. You can hire the staff to do it, then sit back and take in the money it makes. However, like in real estate, that costs more money than just doing it yourself and do you really trust strangers to manage something as important as your income with no oversight from you?
Index funds is really the only true passive income stream I've seen. Passive income is a neat idea. However, being able to earn money with little to no effort is a dream. It's really not achievable for the most part. Warren Buffet is, obviously, a huge outlier and his advice isn't always replicable to the masses. His wealth isn't all that passive either. He was a successful business owner, CEO, and stock broker. He may have made quite a bit from passive investment, but he has also amassed quite a bit by working hard and, you know, running one of the largest multinational conglomerates in the world.