From the Tang writer Han Yu's
《杂说四·马说》:
世有伯乐,然后有千里马。千里马常有,而伯乐不常有 ('There must first be a Bole, and only then a thousand-li horse. Thousand-li horses are common, but a Bole is rare').
伯乐 was the style-name of
孙阳, a famed horse-judge in the time of Duke Mu of Qin; Han Yu uses the image to lament how talent goes unrecognized.