其樂融融

From Zuo Zhuan. Duke Zhuang of Zheng had exiled his mother and swore that he shall not see her until he "reaches the yellow spring (黃泉)", i.e. until he dies. But then he repented this. An officer, Ying Kao Shu, had a solution (《左傳·隱公元年》):

對曰：「君何患焉？若闕地及泉，隧而相見，其誰曰不然？」公從之。公入而賦：「大隧之中，其樂也融融！」姜出而賦：「大隧之外，其樂也洩洩！」遂為母子如初。 [Classical Chinese, trad.]

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对曰：「君何患焉？若阙地及泉，隧而相见，其谁曰不然？」公从之。公入而赋：「大隧之中，其乐也融融！」姜出而赋：「大隧之外，其乐也泄泄！」遂为母子如初。 [Classical Chinese, simp.]

From: Commentary of Zuo, c. 4^th century BCE, translated based on James Legge's version

Duì yuē: “Jūn hé huàn yān? Ruò què dì jí quán, suì ér xiàngjiàn, qí shéi yuē bùrán?” Gōng cóng zhī. Gōng rù ér fù: “Dà suì zhī zhōng, qí lè yě róngróng!” Jiāng chū ér fù: “Dà suì zhī wài, qí lè yě yìyì!” Suì wéi mǔzǐ rú chū. [Pinyin]

"Why should you be distressed about that?" said the officer. "If you dig into the earth to the yellow springs, and then make a subterranean passage, where you can meet each other, who can say that your oath is not fulfilled?" The duke followed this suggestion; and as he entered the passage sang, "This great tunnel, within, With joy doth run." When his mother came out, she sang, "This great tunnel, without, The joy flies about." After this, they were mother and son as before.

其樂融融

1. joyous and harmonious