Yī chǎng lǚxíng ràng tāmen chéng le mò nì zhī jiāo.
A single trip together made them lifelong close friends.
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From 《庄子·大宗师》: four sages 'looked at one another and smiled, their hearts in agreement (莫逆于心), and so became friends.' 莫逆 literally means 'nothing to oppose' — hearts that move in total accord.