It came from a video by Big Think featuring Barry Ritholtz. He was going through a litany of things that stop investors from being successful and was clear that boring DCA investing in diverse securities pays better than managed funds 99.999% of the time at 20 years. In that conversation he mentioned that the old joke was that if you wanted to be successful you should put your money into the market and then do nothing. Like don’t move it, don’t change it, don’t mess with it. You are far more likely to mess things up by trading around than you would be if you bought an index fund and then just never sold it.
For what it’s worth, I’ve never learned this lesson and now that I’m getting to be older I might just have to try it…