Tao Te Ching · Ch. 6

谷神不死,是謂玄牝。玄牝之門,是謂天地根。綿綿若存,用之不勤。

Gǔ shén bù sǐ, shì wèi xuán pìn. Xuán pìn zhī mén, shì wèi tiān dì gēn. Mián mián ruò cún, yòng zhī bù qín.

The spirit of the valley never dies — this is called the mysterious female. The gateway of the mysterious female is called the root of heaven and earth. Subtle and continuous, it seems to endure. Use it; it is never exhausted.

— Tao Te Ching 6

Source: Wang Bi edition (Chinese) + Hanyu Pinyin transliteration