源远流长

yuányuǎn-liúcháng

(1) [of long standing and well established as a stream runs a long course from a remote source]∶因源头远而流程长

(2) [long history]∶指传统或历史长久

言其所学之正, 源远而流长也。 --清·佚名《杜诗言志》