calculus - Continuity of 'derivative of a function'

I just learn one theorem which says If "$f^{'}$ exists and is monotonic on an open interval (a,b),then $f^{'}$ is continuous on (a,b)."I got the proof but now I am looking for one example in which if I relaxe the hypothesis of monotonic-ness then resulting $f^{'}$ is not continuous.I am thinking but unable to get such example. Thanks.