From Double-Tongued Dictionary:
"catch a falling knife"
In English, Money & Finance, Slang
catch a falling knife v. phr. The question for Mr. Marks and others is when to start buying in earnest. Early buyers risk “catching a falling knife,” in market parlance, if prices keep tumbling. But late buyers risk losing out on the best deals and the best returns. “We think we have to catch a falling knife,” Mr. Marks said. —“Finding Potential for Debt in Distress” by Norm Alster New York Times May 25, 2008.
Funny: I have heard it as "never reach for a falling knife," and understood it more as sage kitchen advice. I did not know the investment side of this saying.