I've seen it quoted in several sources. In some cases the authenticity of the quote isn't even discussed, and the quote is presented as-is with no qualifier. In other sources they either refer to it as a "story", a "might be true" anecdote, and in at least one source the author gives a few reasons why it probably was never actually said. I have not found a single source that provides any evidence that it is an authentic quote.

Having said that, it's worth noting that most sources point out that even if it wasn't actually said, it was at least true in spirit.

A source that states it as fact but gives no source.

A source that refers to it as a "story".

A source that calls it a "story", and surprisingly adds an additional part to the story. Allegedly after having broken down sobbing, the General was told by his driver "It gets worse... farther up."

A source that infers the possibility of it not being true by qualifying it with "if the shock expressed by his chief of staff... was accurately reported..."

A source that gives reasons for why it probably didn't happen (I'll let you decide how much weight to give those reasons).