間不容髮

As recorded in The Book of Rites of Dai the Elder, coined by Zengzi:

曾子曰：“……律居陰而治陽，厤居陽而治陰，律厤迭相治也，其間不容髮。” [MSC, trad.]

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曾子曰：“……律居阴而治阳，历居阳而治阴，律历迭相治也，其间不容发。” [MSC, simp.]

From: Dadai Liji

Céngzǐ yuē: “...... Lǜ jū yīn ér zhì yáng, lì jū yáng ér zhì yīn, lǜlì dié xiāng zhì yě, qí jiān bùróng fà.” [Pinyin]

(please add an English translation of this usage example)

Popularised by Mei Sheng (or Mei Cheng, d. 140 B.C.E., pronunciation controversial) in his first letter to a prince of Wu, remonstrating him not to rebel:

墜入深淵難以復出，其出不出，間不容髮。 [Classical Chinese, trad.]

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坠入深渊难以复出，其出不出，间不容发。 [Classical Chinese, simp.]

From: 162 BCE, 枚乘,《上書諫吳王》, translated by Stephen Owen

Zhuìrù shēnyuān nányǐ fùchū, qí chū bùchū, jiān bùróng fà. [Pinyin]

Falling into the deep abyss, it will be hard to get it out again; though between getting it out and not getting it out there may lie not even a hair's breadth.

間不容髮

1. extremely close; hair's breadth
2. (of a situation, figuratively) extremely critical; in imminent danger

Since this idiom was coined to be figurative, the non-figurative sense was attested very late instead.