Recorded in Chapter 60 of
《三国演义》 (Romance of the Three Kingdoms):
某素知刘备宽以待人,
柔能克刚 ('I have always known Liu Bei treats others generously - softness conquers hardness'). The idea traces back further to
《道德经》 chapter 78:
柔之胜刚 ('the soft overcomes the hard').