From Mao Zedong's 1935 ci
《清平乐·六盘山》, written during the Long March. The couplet reads:
天高云淡,
望断南飞雁。
不到长城非好汉,
屈指行程二万 - 'The sky is high and clouds thin; I gaze until the southbound geese vanish. One who fails to reach the Great Wall is no hero; I count the miles: twenty thousand li.' The phrase became a Chinese tourism tagline, especially at Badaling Great Wall.