韋編三絕

From the story of Confucius studying the Classic of Changes so diligently that the tanned leather threads which connected the bamboo slips broke several times. Directly from:

孔子晚而喜易，序彖、繫、象、說卦、文言。讀易，韋編三絕。 [Traditional Chinese poetry, trad.]

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孔子晚而喜易，序彖、系、象、说卦、文言。读易，韦编三绝。 [Traditional Chinese poetry, simp.]

From: The Records of the Grand Historian, by Sima Qian, c. 91 BCE

Kǒngzǐ wǎn ér xǐ Yì, xù Tuàn, Xì, Xiàng, Shuōguà, Wényán. Dú Yì, wéibiān sān jué. [Pinyin]

In his old age Confucius loved to study the Book of Changes and wrote commentaries titled Tuan (Commentary on the Judgment), Xici (Commentary on the Appended Phrases), Xiang (lit. image), Shuogua (lit. Explanation of the Trigrams) and Wenyan (Commentary on the Words). He studied this book so much that the leather thongs binding the wooden strips wore out thrice.

Note that in Classical Chinese 三 may also mean many instead of being exactly three.

韋編三絕

1. (figurative) to study diligently