"When major declines occur, they offer extraordinary opportunities to those who are not handicapped by debt," Buffett wrote in a letter to Berkshire Hathaway's shareholders in 2017, highlighting the argument against ever borrowing money to buy stocks.
He continued: "No one can tell you when these will happen. The light can at any time go from green to red without pausing at yellow."
Source: Letter to Berkshire Hathaway shareholders