历久弥坚 ｜ lì jiǔ mí jiān

Meaning: to become more resolute with the passing of time 

Original: “习近平强调，中朝传统友谊历久弥坚，是双方共同的宝贵财富。”

Source: On September 9, Xi Jinping sent a congratulatory message to Kim Jong Un on the 73rd anniversary of North Korea’s National Day–http://www.news.cn/politics/leaders/2021-09/09/c_1127843420.htm