Sidney Rittenberg was in China from 1944 to 1979, initially as part of
the United Nations' famine relief program. Although he became friends
with Zhou Enlai and Mao Zedong, as well as other revolutionary
leaders, and became a member of the Chinese Communist Party, he was
eventually twice accused and imprisoned as an alleged CIA agent,
between 1949 and 1955, and 1968 and 1977.
Although I haven't found the quote you mention cited anywhere online,
it nicely illustrates the frosty way in which Rittenberg was viewed by
the Chinese leaders after the espionage accusations.
There are potted biographies of Rittenberg here:
An Insider's View of China's Past and Present
The Man Who Stayed Behind (Review)
You can buy a copy of Rittenberg's autobiography, 'The Man Who Stayed
Barnes and Noble: The Man Who Stayed Behind
Hope this answers your question,