天知地知，你知我知

From a story about Yang Zhen in the Book of the Later Han:

當之郡，道經昌邑，故所舉荊州茂才王密為昌邑令，謁見，至夜懷金十斤以遺震。震曰：『故人知君，君不知故人，何也？』密曰：『暮夜無知者。』震曰：『天知，神知，我知，子知。何謂無知！』密愧而出。 [Literary Chinese, trad.]

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当之郡，道经昌邑，故所举荆州茂才王密为昌邑令，谒见，至夜怀金十斤以遗震。震曰：『故人知君，君不知故人，何也？』密曰：『暮夜无知者。』震曰：『天知，神知，我知，子知。何谓无知！』密愧而出。 [Literary Chinese, simp.]

From: The Book of the Later Han, circa 5^th century CE

Dāng zhī jùn, dào jīng Chāngyì, gùsuǒ jǔ Jīngzhōu màocái Wáng Mì wèi Chāngyì lìng, yè jiàn, zhì yè huái jīn shí jīn yǐ yí Zhèn. Zhèn yuē: ‘Gùrén zhī jūn, jūn bùzhī gùrén, hé yě?’ Mì yuē: ‘Mùyè wú zhī zhě.’ Zhèn yuē: ‘Tiān zhī, shén zhī, wǒ zhī, zǐ zhī. héwèi wúzhī!’ Mì kuì ér chū.

As Yang Zhen passed Changyi while he was relocating for his new post, Wang Mi, who was a licentiate (xiucai) from Jingzhou recommended by him, was the governor of Changyi county. He came to visit him, and then brought ten catties of gold for Yang (so as to bribe him). He said, “ your old friend understands you, but you don't understand your old friend. Why ?” Wang said, “It is night now, nobody would know this.” Yang then said, “The Heaven knows, the Gods know, I know and you know. How can you say that nobody knows?” Wang felt ashamed and left.

天知地知，你知我知

1. This secret is only known by you and me.