From Bespoke Investment Group:
...A key level to watch on the S&P 500 is 875. The previous pullback in April ended when the S&P 500 traded down to the level of the March peak. If this decline is anything like the last, we would expect to see support at the peak of the April rally which was 875. If that level fails to hold, the next area of support comes into play at the 50-day moving average (~825).