源远流长 [- yuǎn liú chánɡ]The source is distant and the stream long.; a distant origin and a long development; a distant source and a long stream — of long standing and well established; go back to ancient times; (The river of friendship) runs a long course from a remote source:
•¶ 我们两国人民的友谊源远流长。 The friendship between our two peoples goes back to ancient times.; The river of our friendship runs a long course from a remote source.