Neither labor nor government is the source of all wealth. There is no one source of all wealth. Not all wealth is produced in the economy, but for the wealth that is, the source of that wealth is all of the resources (including labor) that collectively go into the production of that wealth.

Furthermore, we wouldn't produce wealth if people weren't able to buy it. So in a sense, you could say that the source of wealth is spending. And in the sense that the government funds spending (through a basic income or otherwise), the government is the source of wealth.

Depending on your perspective, wealth can have different sources. But figuring out the source of wealth isn't really that important when we know how to maximize people's wealth regardless of its theoretical source.

There's a lecture I really like by economist Steve Keen where he describes his own theory for the source of wealth:

https://www.youtube.com/watch?v=2H1LIr2NMbE

His answer is that energy (a.k.a. the sun) is the source of all wealth. That's certainly a reasonable way of looking at it that you can't really argue with. But it doesn't tell us how to improve human prosperity.

My favorite quote from the lecture:

"The theory of production has got nothing to do with the theory of remuneration."

Basically this means that how much you get paid has very little if anything to do with how much you contribute. And that's exactly the way it's supposed to be. Markets determine the prices of various forms of labor and that's fine. Let supply meet demand and don't worry about whether wages happen to line up with individual incomes that are optimal for the economy.

Getting individual incomes to prosperity-maximizing levels what basic income is for.