Watch Art Cashin: Two numbers you MUST monitor

Cashin says: Watch for yields above 2.9%

Art Cashin says there are two important numbers that investors need to keep watching.

Cashin, UBS' director of floor operations at the NYSE, told CNBC's Bob Pisani around midday that if the yield on the U.S. 10-year note goes above 2.9 percent "then I think you can get some perhaps serious selling showing up here."

The other number is the S&P 500 stock index. "I think the viewers want to watch, if we get extra pressure here, the S&P, 1772-ish. Important support. If they break that there might be a gap to the downside."