ngưu tầm ngưu, mã tầm mã

Etymology
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The idiom is from Zuo Zhuan (左傳; Tả truyện), where Wu Qi (吳起; Ngô Khởi) during a battle lost his horse and it ran into a herd of wild buffalo. This is a common explanation as to why this idiom has a negative connotation.

Idiom

 * 1)  the water buffalo seeks the water buffalo, the horse seeks the horse.
 * 2)  bad people often find other bad people
 * 3)  to plot something shady